I live at 9000ft and will be running trails that can max out at 13,000+ So I need something that can deal with the altitude, can climb steeps well, rugged suspension, and can take a beating.

There's a Twister HH ss250 up for sale now in my area for $2,800 on craigslist. Don't know if it's enough for my needs.. plan on pretty hard riding in all conditions & challenging terrain (think CO and Moab, UT). But now I'm drooling over the Kinroad GK 2A. See it and you can drool too.. www.kinroad.net/Show.asp?ClassID=41 Sticker shock, but darn, smokin'!!

So being a newbie, I'm not quite sure how big to go on engine size (how much difference from a 250 to 650 or even up >1000cc?), can certain mods really help makeup the difference in engine size that much? What mods should I be thinking to deal with altitude?

I don't know a whole lot about the wrenches & grease, but can't wait to learn and get going!!
Thanks for any advice!

The 800cc buggies and larger are fuel injected which would help your elevation changes. I do not know any 650cc or smaller buggies that do not run a carburetor.

If you are looking at good suspension, the Hammerhead may not be your best choice since they use a strut-type suspension up front. You should look at the Blade 250XTX, Kinroad Pheonix 300 and any of the Joyner buggies.

Used buggies go for about half of new, so you can get quite a bit more buggy for your dollar if you don't mind someone else taking the depreciation hit.

maybe the joyner 650 or 800 but for the 650 u need to deal with the carb and tune in with the jets in the carb