You've touched on a lot of issues, each of which could generate a lengthy thread in and of itself. Unfortunately, I don't know of any Web resources that provide clear and succinct descriptions of the relevant points, so I'll just provide a few thoughts on each of the issues you've raised....
I have one question - which type of enlarger (brand and model) should I buy. I have decided to buy used (I think). Are there models I should avoid (problems to look out for)?
There are so many different brands and models that it's hard to make sweeping statements. IMHO, more important than the brand you get is its condition, at least in terms of the quality of a used product. Look for something that's built sturdily -- for instance, with a thick metal column rather than a thin hollow metal column.
I can say this: I've owned two enlargers, both bought used: a Durst C35 and a Philips PCS130 with PCS150 control unit. The Durst was a bottom-of-the-line model with cheap construction and certain limitations that I found, well, limiting. The Philips is a much better unit, but it's a bit of an oddball and it's been out of production for two decades. If you had a choice between only these two I'd say you'd be happier with the Philips, not least because it can handle up to 6x7, which you say you use. Even for 35mm only, though, the Philips is a much sturdier and more capable unit, even if it uses rather expensive bulbs.
I am printing 35mm and medium format (645, 6x6, 6x7) B+W prints.
The oft-quoted rule of thumb is to get an enlarger that can do one size larger than your largest current format. That said, if available space is an issue, you might not want to go that route. Some people also use different enlargers for different formats -- a large format enlarger might not be ideally suited for 35mm, for instance.
I would like to print large - is 20 X 24 doable?
Most enlargers can print up to at least 11x14 (inches) on the baseboard. You can usually rotate the head to project on a wall or unmount the column and mount it backwards to project on the floor to get larger prints. I'd expect 20x24 should be possible with most or all enlargers that support either of these features. Since this is important to you, be sure to check on this feature for any enlarger you consider. FWIW, my Durst C35 did backwards-mounted floor projection and my Philips PCS130 does both this and wall projection.
I do not have a fortune to spend - how much should I earmark for the enlarger?
In the used market, a good enlarger capable of doing up to 6x7 can be had for under $100 if you shop carefully. I paid $50 (plus $24 shipping) for my Philips PCS130 with PCS150 control unit a bit over a year ago, for instance. Mine only came with negative carriers and condensers to handle up to 6x6, though. I just recently acquired a 6x7 condenser set for about $30. The price goes up for certain feature-laden models or for large format enlargers. There's also a certain randomness at work -- I've seen posts from people who've gotten excellent enlargers for free, and others from people who spend several hundred for less than I got a year ago.
As far as I have seen - I need a 50 and an 80mm lens for everything except the 6x7 (what size lens does the 6x7 need).
The general rule of thumb is to use a lens that's the same focal length as the "normal" lens for the film format you're enlarging. For 6x7, 90mm is often used. I've got a cheap 75mm lens for 6x6, but 80mm is often recommended for that format. For 35mm, a 50mm lens is the most common choice.
It does not have to be pretty - but reliable and give me very good results that I could possibly display or put in an show (I may be stretching here) and be fairly hardy (I will not be buying a new one anytime soon.)
Any enlarger is capable of producing good results, although you might need to invest some time in aligning it or perhaps repairing problems (electrical faults, light leaks, etc.) if it's in bad enough condition. Cheap (as in cheaply built) enlargers may be susceptible to vibrations, which will blur your photos, but even they can do OK if they're put in stable enough environments. Better enlargers are more resistant to vibrations and will be easier to use. For image quality, pay attention to the lens, but be aware that you can ditch the lens that comes with the enlarger in favor of a better one if you don't like what comes with the enlarger. Typically, 6-element designs from Nikon, Schneider, and Rodenstock are recommended, but some other manufacturers make lenses in the same league.
What are the difference between condenser/cold light heads?
These are two different factors:
- Light source -- cold light heads use fluorescent tubes as the light source. I'm not very familiar with these, but I believe they're most common on large format enlargers. The smaller enlargers I've used, such as my Philips, generally use tungsten or halogen bulbs.
- Condenser vs. diffusion design -- Condenser enlargers use optical condensers (large lens-like objects) in the light path to spread the light evenly over the area of the negative. Diffusion enlargers do the same job using a diffusion apparatus -- typically a box with a white interior that bounces the light around, keeping the bulb out of direct view of the negative and lens. There are endless debates on which is better. The conventional view is that condenser enlargers produce sharper prints, but at the cost of greater visibility for scratches and dust. Condenser enlargers also typically produce slightly higher contrast. Most color enlargers (see below) use diffusion designs, whereas both types are common in B&W enlargers. (My Philips is an oddball in that it's a color condenser enlarger.)
Are they both B+W - is one better than another?
Any enlarger can be used for B&W printing. Not all are equally well suited for printing with VC paper or for color printing, though. The issue is filtration: Many older enlargers lack filter drawers, which means you've got to use under-lens filters for VC or color papers. Most more recent enlargers have filter drawers. Some enlargers have VC or color heads, which incorporate magenta, yellow, and (for color heads) cyan filters. (A few models, including my Philips, use red, green, and blue filters for additive color rather than cyan, magenta, and yellow for subtractive color.)
Overall, I'd recommend getting an enlarger with a color head, even if you don't intend to print in color. You can use the magenta and yellow filters to adjust the contrast of VC B&W papers, and you'll have the option of doing color if you take an interest in it in the future. If you're sure you'll only do B&W, you might consider an enlarger with a VC head with magenta and yellow (but not cyan) filters or separate blue and green light sources (some cold-light heads are built this way, or so I understand). At the very least, be sure whatever you get has a filter drawer and price filters before buying (if the enlarger doesn't come with them). If you're certain you want a condenser enlarger, you'll find few with color heads. (Some models support multiple heads, so you might get an enlarger with a B&W condenser head and a color diffusion head. You could still use the color head for B&W enlargements, or even the B&W head for color with appropriate filters.)
Other factors to consider:
- When buying used, be sure to buy an enlarger with all the little things you need, such as negative carriers and condensers or mixing boxes for the sizes you want to use. If you buy an oddball enlarger, extras like negative carriers or condensers can be hard to find. If you buy a popular model or one that's still in production, you'll be able to find such things, but they may be expensive.
- If possible, get a complete darkroom "kit" with trays, tongs, an easel, a safelight or two, etc. These extras all add up in cost remarkably fast. On eBay, complete kits often go for little more than the enlarger alone.
- Check the prices of bulbs. Some enlargers (such as my Philips) use expensive or hard-to-find bulbs.
- Read descriptions in eBay auction ads, catalogs, etc. You may learn of a feature that appeals to you.
