I have shot the 645AFD system extensively and have access to one presently. The lenses are superb and the handling of the camera is quite nice with not much more weight or heft than an autofocus 135 system camera.
Now the af isn't going to preform like a modern 135 size slr/dslr but is VERY accurate if a bit slow if you are used to modern 135 af.
645 is a nice format IMO and there is plenty advantage resolution wise to make it "worth it" over 135.
Good luck with the sale.