I'll jump in and answer the SS v. M engine question since i think i know..
the SS has different cams that gives you more power up top.
having an old 900SS and a new M900 in the garage, I can verify this with my highly tuned and calibrated Ass-Dyno.
the Monster feels a little stronger on the bottom, but runs out of steam way before the SS.
I wouldnt sweat the mileage if you're willing to do the maintainence. my SS has 45,000 and has no problems. it's ready for some engine work...like a piston/ring kit and valve guides/seals. not a big deal.
if you like the idea of hopping the 750 up, there's lots of parts available.
if my wife would let me hop up her M750, and i had a buncha money, i would do:
hi-comp pistons, 39mm flatslide carbs, dyna coils, light flywheel, and maybe have some headwork .
of course, it may be cheaper to get a bigger bike or engine, just depends if you're looking for final result, or if you like the tinkering process.