Basically a medium format camera is not all that different than a 35mm camera. Don't be intimidated. It's easy!
Shooting 645 is definitely a step up from 35mm. If you like square then 6x6 is the way to go. If you don't like square and are going to crop then you would be better off shooting 645 in my opinion.
If you like rectangles and want the best quality then 6x7 is what you want. The disadvantage to 6x7 is the bulk and weight. Another disadvantage to 6x7 is that some 645 cameras have autofocus, matrix metering, and auto film advance etc. if you like that kind of stuff.
Buy what you like and can afford and don't worry about the critics. After all it's all about the image, right?