My research methods are relatively simple, but are highly dependent on what type of build I'm working on. I only build WWII armor (for the most part) and my builds will vary from a specific vehicle from a specific unit on a spectific date to a generic vehicle from an unknown unit on a general date. The research required for each type of build is obviously quite different. In general, I don't know what vehicle I'll model before I decide to build the kit. One day I'll say to myself, "I want to build a Sherman" and then decide which variant and which kit to build. Only once the kit is selected will I start looking for specific vehicles/units/time frames.
Research usually involves looking through my own reference material first - both books and saved photos from online sources. Depending on the build, I'll then do varying degrees of online research. Generally, this is more than adequate to get the information I want on the build. If, for some reason, I can't find sufficient information on a subject to do a build from these sources, I'll change the subject rather than beat myself up looking for information.
For me, limiting myself to a specific subject before I even think about a kit feels too much like "contract work" and can take away from the fun in the hobby. I know many modelers who love to pick a subject first, check out appropriate kits, look for (or develop their own) decals, do a ton of research, then start the build - trying to replicate this one subject perfectly. This is their way of building models, but it's not the way I prefer to work. To each, their own...
Good luck on your TANG project. It sounds like a great series of builds.