check graflex.org for the speed graphics---the 3x4 speed is smaller than the 4x5 but you can get the 4x5 graflock back and graft it on---it's a little work....alternatively, it may be easier to just "plug and play" with a cheap 4x5 graflex with graflock back already on it....if you've never worked on cameras before or don't want to bother, then this is easiest and no time wasted, etc....
anyways--see how you can mount ANY lens onit that you want--you put in a polaroid 405 back for the 3x4 polaroid/fuji instant and you're in business---so if you want to mount hasselblad lenses, you can shoot with hasselbladlenses and get the full coverage of them lenses not cropped to 6x6--or you can put on pentax lenses----whatever...some of these may cover the full 3x4 frame size
see...medium format with polaroid really aint worth it---even if you don't mind losing so much image area, they are cropped in such a way to look ugly---like they're not centered with unevenblack borders---
in any case if you use the larger camera, you can plug right in to shooting full frame when you get sick of the cropping boo yah....
aanother alternate---shoot any medium format of your choice and then ENLARGE these to polaroid to make prints....
I was doing this with great success--you shoot slides and then you have the opportunity to crop/etc the final image on the polaroid in the darkroom--you basically print on the polaroid instead of having a whole wet darkroom--then you can even shoot 35mm--with the small enlargements to the 3x4, you will lose no quality at all--that is...shooting medium format to go to polaroid--your basically wasting the medium format detail--polaoid/fuji instant is much less detailed due to the diffusion inherent in the process.
i, myself, many years ago started with medium format/polaroid back and quickly upgraded to full frame polaroid with the larger camera--a speed graphic---that was the "gateway drug" to larger and larger cameras.....