I have to agree--645 has never really appealed to me much as a format. If one is going to have only one camera system, and wants a compromise between the light weight and versatility of 35mm with a bigger neg, then 645 makes sense. 645 is definitely an improvement over 35mm, but if you're going to keep 35mm, you might as well go to 6x6 (if you like the square format) or 6x7 (if you prefer rectangles) and really take advantage of the larger camera, using 35mm for what it does well, and MF for what it can do.

There are lots of options, and have been quite a few threads about getting into MF, so you might read through them and see if that helps narrow it down.