I would highly recommend looking around for a Polaroid Automatic 450 Land Camera. It features a coupled rangefinder produced by Carl Zeiss, and as near to a full set of exposure levels as one can get with any of the non-professional (180, 185, 190, 195) Automatic series... I picked mine up at a flea market for five bucks, and it's my favorite Automatic so far.
As to reasons one might want a Polaroid in addition to a regular medium format camera, three things to consider: First, Polaroid 669 and 667 positive films allow all sorts of cool manipulations, like emulsion lifts and image transfers. Two, Polaroids can give you a good idea whether that perfectly lit shot is really perfectly lit, prior to taking a photograph that you want to be
just right. And finally, you won't really care when your Automatic series Land Camera gets dropped in some filthy pit somewhere, being that you can just get another one for ten bucks or less. Same doesn't apply to your Rolleiflex
Yeah, I'm a bit of a Land-o-Phile, but c'mon, Polaroids are damn cool.