It all comes down to what you want it to do and budget.
The 645E does not have interchangeable backs I think, which you may find limiting. I have an RZ67II whiich is great, but also great BIG and at 2.5kg certainly slow and ponderous.
Have you thought of going in cheap to start with, such as a second hand Rolleicord V. I have one and it is very sharp stopped down, wonderfully soft wide open! Fixed lens and no interchangeable back, but if you are into simple stuff (and prepared to be disciplined) it does enough. I would not be too wary of ebay. Feedback is everything. If they have loads of 100% feedback and the item is described clearly, unambigously and in detail, then your chances are very good. Poor feedback with vague descriptions...be wary!
My rolleicord cost £100 and is mint.
Good thing about the 645 Mamiya is that used lenses are cheap and plenty. Bronica prices have completely crashed, so are worth a look as this must have hit used prices. A new SQAI was about £1100 about 1 year ago...now £699. I've had one and there is nothing wrong with them!
Oh, just bite the bullet and get big beautiful view camera and dont prolong the agony any longer.