Welcome to APUG! Unless you have loads of money for medium format go simple at first.
A nice Yashica will only set you back $US100 or less. Granted, it won't have interchangeable lenses or backs but you will get a lot of fun for little money.
And you will also have a continuous burning desire for stuff that will allow you to do other things, which that one trick pony can't.
So my advice: don't waste $ 100 on cheap fun that you could also have invested in a sensible camera.
A sensible camera that will give you even more fun.