The Mamiya 645E comes with a fixed, prism finder. You cannot change the finder. Therefore, a waist level finder is not an option.
There is a built in meter in the finder.
In addition, unlike the M645 Super or M645 Pro or M645 ProTl, the 645E uses film inserts only - you cannot use interchangeable film backs.
The 645E allows use of a Rapid Winder - a thumb operated, 35mm like winder. There is no power winder for that model, although some apparently have had success mounting the power winder for the older, M645 1000S to it.
All manual focus Mamiya M645 lenses will work on the 645E (as well as on the Super, Pro and Pro TL models). They all use the same film inserts as well.
If you want to look at the M645 Super, M645 Pro or M645 Pro Tl, most of the accessories are interchangeable between them, but not all. Some of the AE prisms and winders differ, as do some of the smaller parts. As an example, if you buy the Mamiya left hand grip, the connector between it and the electronic shutter release socket is different between the Super and the Pro (and it is a PITA to figure which is which).
The Super is often very inexpensive, but it is the oldest of that range, and the gears (and probably a few other parts) were strengthened when that model was replaced with the Pro.
I own two Supers, and one Pro. I like them all. I could see replacing one of the Supers with a 645E, because it is lighter, it has a built in diopter adjustment, and I'd like to have one with the thumb operated Rapid Winder.
Hope this helps.
Matt
P.S. The Mamiya America website has lots of useful information and a pretty good User Forum. You can find it here:
Mamiya America
One warning though, it is Internet Explorer oriented - if you use Firefox, you must install the User Agent switcher to have it mimic IE