I own the 645E and the M645. Both use the old manual focus lenses. The 645E has a fixed prism on the top. It can't be removed, can't slap on a waist finder. The M645 will use both waist finder and prism. Both the 645E and M645 use the inserts for film, they do not have interchangeable backs. So there are a number of differences between the models and you need to figure out what features are important to you and which model has them.
For instance, if you want auto-focus you don't want the 645E or the older M645. If you want a built in light meter you don't want the M645 I have to use a hand meter for it or locate a prism that that has a meter in it. If you want interchangeable backs you don't want either the M645 or 645E.
It might help you to just write down the various models and research them on-line. Write down the features each has so you can make a smart selection?