+1 on the ETRSi comment, too. 95% of the time I shoot my ETRS. I got my original rig for about $125, with a body, 75mm, 120 back, and prism. Add in a 50mm f/2.8, and a 150mm f/3.5 and a speed grip for 35mm-like handling and you're under $300 for everything you could need. A Nikkormat or other nice Nikon body, 50mm, 28mm, and 100ish lenses of good quality and you're looking at $300, too.
I find that my handheld shots on 645 with 400 speed film are about equal to my average tripod 35mm shot with FP4. With FP4, 645 is actually a huge step up from 35mm, and apparent to me (tonality-wise) at an 8x10. Plus, I find 35mm reels tedious and difficult to load. 120 film on the other hand is very easy to work with in the darkroom. If she's trigger happy on digital, MF might make her slow down a bit at first, making the cost difference minimal when you figure in mediocore shots and such that can happen if you're trigger happy with 35mm.