Look for the Yashica Mat. Is a 6x6 square format. Go for any Yashica models higher than the D. It has better lens. Is a Twin Lens Reflex camera. One lens for composing and viewing. The other is for taking the actual shot. They are OLD, more than 70 years old. A CLAed unit would cause you around $150 to $200. Most of them on Ebay are beatup or partially working. They can be had for around $80. They may issues considering the age of these cameras (maybe wrong shutter speeds, scratched up lenses, or partially broken). I bought mine for under $80. It took me a while to learn how to refurb it myself (taking it apart completely and to clean and lube the inners and to refoam the light seal). I later enhanced it by painting the film chamber with some flat, anti reflective paint, and adding a better ground glass for focusing.
You can shop for the Mamiya 645 lineup. A bare system would still cost you around $300. Checkout Keh.com too. Unless you are buying a TLR camera, most medium format system would require the body, the film back, the waist level finder, and at least one lens. Most of them don't have a lighter meter. The Mamiya 645 system has an integrated film back with the body.