You'll be shelling out at least $5K plus for an extended warranty (if you can even get one at all) and the ac upgrade will cost you at least a $1K, and I have heard as much as $4K. You definitely need to shop around for both...
Regarding the A/C upgrade either one of these guys should be able to lead you in the right direction:
Chuck Kopelson who runs
www.flashoffroad.com
Dave Breggin with
www.BlueHummer.com
Regarding the Extended Warranty:
Dave sells it at his website-
http://www.bluehummer.com/exwa.html
And Ben actually owns the company:
http://www.exoticwarranty.com/
Try these websites:
To Possibly Purchase the upgraded A/C:
http://www.real4wd.com
http://www.calgoldproducts.com
Explanation of the A/C Upgrade:
http://www.humvee.net/hid/interior/acup.html
http://www.humvee.net/hid/interior/dbac.html
Hopefully this information helps. I'm right with you as I continue to look for that perfect Hummer Wagon...
Thanks.