You could go with the Revell AT-AT easy kit, but it needs some worl for the head area at the very least. It is around 1/72 scale. On the other hand you could find one of the old MPC/AMT/ERTL AT-AT kits on ebay that are priced pretty reasonably and can usually be easily found. That kit is more accurate, but is is a bit smaller at around 1/100 scale. On the plus side, it came with 2 snowspeeders and 2 gun turrets as well (unless you get the later one that only had a movie print)
Anohter one worth mentioning is the F-Toys AT-AT which is supposed to be around 1/144 scale.