It is a good recommendation to buy what you want up front, since it will cost time and money to make the upgrades later. It is almost always cheaper and faster to get what you want from the factory, and it will be done by experts. However, if you are looking for an opportunity do some experimental gunsmithing then that is a different question.
The Rock Island Tactical has all the mods you want, but I don't think it comes in stainless. In general, I like stainless guns. I have an S&W 686 and S&W 4506 both in stainless. I also have a Colt 1911 Combat Elite with stainless frame and blue slide. I would rather the Colt were all blue since, to me, blue looks better on 1911's.
Rock Island and Springfield both have very good customer service reputations, as do S&W and STI. The $899 for the used Kimber is high. The TLE II is near the baseline model in the Kimber lineup. I'd easily recommend spending an extra $200 for a gun with no rust.
Rock Island and the STI Spartan both have the major parts (frame, slide, and barrel) made by the same company, Armscor, in the Philippines. Armscor has an excellent reputation, just know what you are buying. STI does have other pistols in their line-up that are completely made in Texas if that makes a difference to you. One thing to think about with the STI Spartan is that it comes with an adjustable site which has a different cut on the slide than the fixed sites. It is a big advantage to some users, but "most" 1911's don't come with an adjustable site which will limit your options down the road if you don't like them and want to change them out.
Another thing to consider is whether you want a firing pin safety (FPS) or not. Most "modern" pistols have a firing pin safety, but not all 1911's have them. Many 1911 fans refuse to own a 1911 with the firing pin safety. The RIA, Springfield, and STI don't have a FPS, while S&W and Kimber both have a FPS. Interestingly, each of of their FPS designs are unique in the 1911 world. Most other manufactures using a FPS, use the Colt Series 80 design.
There are many nice 1911's out there and many quality 1911 manufacturers. However, while they are all 1911's, there are also many different features used by the different manufactures. Some have FPS, some have full length guide rods, some have ramped barrels, some have bulled barrels, some have external extractors. You need to make sure you know what you are buying and whether those are the features you want.
1KPerDay wrote
I thought Rock Island wasn't making 1911s anymore.
I believe you are thinking about Rock River Arms which suspended 1911 manufacturing about a year ago to concentrate on AR production.
Jim