1. Can't help you there, I take a blue foam square or nothing. Have been known to throw down my rain jacket to sit on the grass or a rock. I b'lieve NeoAir makes a sit pad, might check that out.

2. Why not a poncho with tie out loops? Plenty of those around. Tie off the hood and suspend between trees with cord, or use a long stick or extended trekking pole for a center pole.

3. Lots of options, the Garmin 60csx is the one I'm most familiar with as our SAR team uses them. It's one of the more fully featured ones, does everything you want plus some, and I've seen it on sale at Cabelas and REI for around $200 (full retail is listed to be $500). It works pretty well, have yet to lose a satellite signal (common complaint with a lot of GPS units). It's a battery pig, but I have yet to meet a GPS that isn't.