Rick is right about get ready for the flood of answers. I am sitting here though, actually giving this some great thought. First opinion......don't waste your time on 35mm, get your money into your 645 system and be done with it. You will probably come to be greatly dissappointed by the small little negative compared to what you are normally used to.
Then, I was going to give you the standard Nikon FM, FE, or FA answer but even these cameras are certainly commanding good amount of money these days. So my recommendation really goes out there....N75, N80 or N90. That basically represents the cheapest consumer AF camera to slightly higher cost models. You would get AF and all the bells and whistles and the large choice of manual and AF lenses (don't use G type lenses though). I think I got a new in the box N75 about a year ago for $75. Get yourself a 50mm f1.8 or f1.4 (manual or AF) lens and you would have nice camera. The autofocus will spoil you though......well, maybe not, since you are not exactly doing "action" with your Hassy or you LF equipment, you won't care about it if you are taking pictures of your "normal" non moving objects, but the AF will certainly allow you to start playing around with areas you haven't probably played with....stuff like street photography and the like. Just be clear, 35mm is no substitute for MF or LF photography.
Love to know what you decide to go with.