The Revell AT-AT mainly needs work on the head to add the armor plates. It may sound like alot of work, but it is pretty easy. If you do nothing more than just that, you will have a nice sized model of it. The added benefit is that it is pretty spot on for 1/72 scale so it fits right in with the Bandai and Fine Molds kit lines. With its recent re-release, this is probably the easiest and cheapest kit to get. If you want to go to town and make it more accurate it still provides a decent starting point.
To have some speeders to go along with it, you can get the RetroSF 1/72 snowspeeder, or the DiAgostini Die Cast one (it is maybe 1/70) or the Revell Pocket speeder (it is larger, but if you prefer the size of the speeder to the AT-AT in the foot crushing scene it fits)
If you are looking for something larger, there are several studio scale "garage" kits (around 1/48). Some require more work than others and all are fairly expensive. Although they are not necessarily more accurate either. These all pair nicely with the Fine Molds or Bandai 1/48 snowspeeders
The old MPC/AMT/ERTL one is a bit smaller than the Revell one and is somewhere around 1/110 and 1/144. it pairs nicely with the F-Toys 1/144 speeders since they are a little oversized for 1/144 anyway. This you can build fairly easily right out of the box and have a great looking model, or you can go to town accurizing it. This might be harder to get since it is out of production and sometime it is priced like it is a collector's item on e-Bay. There have been many different packagings of it over the years, but if the brand is MPC, AMT or ERTL, they are all the same toolings