Well, the most recent version of OS X would be the best place to start, however, that requires a new machine, which would not be cheap. Cheaper Macs would be older, and not able to use the newer OS versions!
So your first two criteria are at odds.
You can find the system requirements for each OS version on Wikipedia.
EveryMac is also a good site for learning about what models have what specs, processors, etc, and what OS versions they support.
If you are looking for a very cheap second-hand machine, I would suggest you find a Mac that is 3 to 4 years old. That means it will support the latest OS 10.8 (Mountain Lion), or the previous one 10.7 (Lion) at the least. It will therefore still have a bit of life in it, but be considerably cheaper than a new machine. Have a look online at eBay etc. If you can afford a newer one, then try a newer model.
You need a $100 adaptor? From what to what? For what?