I would like to do some medium format work. What is a cheap but good camera to start with. I have several 120 cameras but which would be a good camera to use
I would say a TLR (Twin Reflex Camera) body is probably the most affordable medium format system available.
You can find a Rolleiflex and Seagull Camera bodies for under $300 USD.
Check out the TLR tagged images from flickr.
I recommend a Kiev 60 (upgraded by http://araxfoto.com/). It is cheap compared to Mamiya and Hasselblad, etc. and yields excellent results.
For $250? I wish you luck, but I think you're in tough. Craigslist is a good place to start, though, but anything in that price range is likely to be a bit iffy. Don't forget that lenses would be your next stop and MF lenses can get expensive. I previously would have suggested digging up older Pentax 645 or 67 MF cameras, but with the arrival of the 645D, you may get the body, but the lens prices are going to jump and that will be where you really hit the cost.
If you want something most similar to an SLR at that price, you will be looking for a 645 camera. You may be able to find a Mamiya for that price if you're lucky, but you can definitely get a Bronica for that price. Bronicas are tough cameras without too many fancy features, but they are cheap and the lenses are good.