if you want (reasonably) fast autofocus, try the Hassey H series of 645 cameras... focus not exactly as fast as 35mm format cameras but faster than Mamiya 645AFD... the Sinar Hy6 looks interesting as well but haven't tried that yet... can't wait to visit Sinar booth at imaging expo...
but the Hassey H series and Sinar Hy6 with digital backs (new, not second hand) are going to set you back in the mid to high 5 figures...
you can definitely use for location as well... just make sure you have plenty of CF cards or plug in to a laptop (or some brands can plug to external harddrive storage) for large amounts of shooting... most newer backs can shoot decently fast, like less than 1.5s per shot, and some backs like the PhaseOne P30 can shoot 1 shot per second continuously, so should be no problem for fashion/model/commercial shoots...
incidentally, there is a digital back for SLRs... the 10Mp Leica
Digital-Modul-R that fits onto a Leica R9 SLR... but it has since been discontinued...
by the way, if you want to hand hold the Hassey H series, train those arms... the body itself is like 2kg, and unless you use the 80mm or 100mm which are relatively light, you are looking at 3-4kg in total with lens... doable sure but tiring... no, there are no VR or IS or OS lenses... and be prepared for 95mm polarizing filters that cost over $500 with some lenses...