In the US, would look at either Beseler or Omega enlargers, simply because parts are easier to find in the US. My personal preference between the 2 is Omega, only because that is what I started on in high school and am familiar with.
If you have the vertical clearance, get the taller XL chassis. That will let you print bigger prints on the baseboard.
Download the manuals and study them. When you look at an enlarger, you want as many parts as part of the package. Buying individual replacement parts can get expensive, turning a good deal into a not so good deal. Things like; negative carrier, lens boards, lens adapters, condensers, etc, etc. all add up and increase the total cost. The more formats you want to set up for, the more the total expense because of the need for format specific stuff like negative carriers, lenses, separate lens boards/adapters.
I have found that the recommended lens for a specific format sometimes differs based on the manufacturer. For 6x7, some say 80mm some say 90mm some say 100. You should check the manufacturers web site for that specific lens. If you get an XL chassis enlarger, then using the slightly longer lens should not be an issue.
As for prices, if you are patient and SHOP well, you should be able to find a good 80mm lens for about $60 USD. The 90mm lens is harder to find so you may have to either drop down to a 4 element lens, or spend more time and $ looking for the few 6 element lenses. The 100/105 would be about $75 USD. If you did your enlarger shopping well, the lens would be part of the enlarger package.
The "big 3" lens manufacturers are:
- Schneider: Componon-S 80, 100mm (caution, Schneider has several models where the names are confusingly similar; Componon-S (updated 6-el), Componon (older 6-el), Comparon, and Componar (4-el))
- Nikon: EL-Nikkor 80, 105mm (not the 75mm lens as that is a 4 element lens)
- Rodenstock: Rodagon 80,105mm (caution, some of the Rodenstock lenses have similar sounding names)
However, none of these make a 90mm 6 element lens.
Here are the only 90mm 6 element lenses that I know of:
- Besler ColorPro
- Fuji-Fujinon EX and EP
As others said, watch Craigslist. I have seen entire darkroom packages for attractive prices, or even free where the seller just wants to get rid of the stuff.
For accessory items; department store and dollar stores are a good source for some of them at low cost.