1) Good idea - I'd guess high rather than low on platform. A lot easier to load something onto a lower platform than you're used to rather than a higher one than you're used to. 5' or so would be good IMO.
2) Get a loading pin and clip from Ironmind, a chain to attach them to (doesn't need to be too fancy) and add weight. The biggest challenge of most wheels (besides being fucking heavy) is that they'll swing back and forth; enter chain suspension.
3) I'd say 500 as well. If you want to make the tire heavier, drill a few holes around the interior and put on some heavy bolts to put plates on as pictured in the recent article on the Air Force Academy gym. Or get a bigger tire. They're free.
4) I've looked into buying a chain - it IS unreasonably expensive unless you can get it scrap or know someone who works at a dock and will give you an old one. Good luck with that option I agree that a sled is more practical. Gojira had a thread a while back about building one, or you can get one for 75$ (shipping incl) by searching for strongman sleds on Ebay.
5) In addition to the link provided, search google for "tightwad strongman implements." I've used JV's design with success.
6) Anything less than 10 tons won't be difficult for anything other than GPP if you use the standard rope and harness setup. I say this seriously - saw a 114lbs woman pull a 14 tonner 50 feet. If you're pulling backwards (moonwalk style) with just a rope attached to a bumper, a full-size pickup may work for now if you load a lot of stuff in the bed, maybe pull it up a slight incline.
I know I mention the site a lot, but the Power and Bulk has a big strongman community, you may try posting there or the Anvil as well.
Hope this helps,