The Mamiya 645 is a good budget system choice for the format. Strong sales means there's a reservoir of decent quality bodies, lenses and accessories at good prices. The only must-have accessory for me was a motor grip that greatly improves the iffy ergonomics of the body. But don't you already have a 645 system, albeit a rangefinder? A contrarian option is to go up to 6x7 and to look into the Mamiya RB67--big but remarkably versatile with good lenses. Got one+a couple of lenses last summer and can't stop shooting it. Good thing my Mamiya 645 and Bronica SQ-B kits aren't the jealous kind.