The reality-check response:
I live in a major US city (Dallas, TX) - but Dallas is a little "new" compared to the northeast, where there may be many more enlargers from the 50's to the 70's on the used market. But for the DFW area, Craig's List is pretty hit and miss, and you might spend months searching for the right enlarger to come along.
If you live in a big city, hunt for used photo stores. We have a great one here, and they also carry the whole ilford line of paper and chemicals… I'd guess any big city has **something** along those lines, sort of a photo-centric pawn shop at the least.
Then there's eBay, where you can't inspect the enlarger… on the other hand, many people sell decent enlargers for a hundred bucks, and if they're regular sellers the shipping might only be $50-$75. Less savvy shippers will charge $100 to ship an enlarger.
My un-scientific observation: there's a lot of Beselers out there. Skip the Printmakers and Cadets - they're cheap student enlargers. Also skip the printmaker 67, which is a 35 with bigger condensers.
Entry level "serious" Beseler is the 67, which comes in a variety of single-column formats, condenser, dichro head, and the rare-ish B&W filtering head. It's a sturdy enlarger, limited alignment controls for going bigger than 11x14 though. There's also a fairly rare model where the 67 head was placed on the 23c body. All versions do up to 6x7. Lens boards and neg carriers show up on eBay - the lens board is tiny and can be hard to find if not included.
Next up and very very common is the 23C, which started life as a blue-painted unit and evolved into the 23C III XL, which is still fairly common and a very sturdy beast. EBay is full of neg holders and lens boards for it. Seems to be the most common used enlarger out there??
Beseler dichro color heads come in 2 eras, based on the power supply - early units needed an electronic power supply which will eventually die and are hard to find. Newer ones have the PSU built-in. If the head has a standard edison power plug and the name plate says "110v", it's the newer one.
As for 4x5 - I find them to be rare beasts on the used market. I'd like one, but I rarely see 'em, and they don't get marked down to "almost-giveaway" status like the 6x7's. Lens boards and neg carriers could be a pricey hunt for many 4x5's.