[KeithT712]

Welcome!

Definitely post up some pics!

Jsfucttruck covered a lot of that..

I second the tranny service although i didn't think the 4l80e was in the square body suburbans but i could be wrong as far as i knew its either a turbo 400 or a 700r4, it sounds like it really couldn't hurt much at this point, and looking at the fluid coming out may give you an idea of how long you really have left. Could try some sort of additive if you felt like it.

Same goes for the coolant temp sensor, I think there may be 2 of them also (not 100% sure)

as for the tailgate issue is kinda 50/50 like you said could take the gamble on a junkyard gate or mess with it and try replacing the motor yourself. Personally I've never done one my self only heard the stories

I think theres a door alignment tool you can buy (depending on how bad it is) thats the one I've got but only used it once or twice

I've got 4" tuff country E-Z ride springs going on my suburban front and rear that are supposed to be a decent ride while still being able to haul/tow when needed, I got them along with a majority of my parts over at Off road Design

Honestly kind of sounds like the bigger problems were covered in most of these questions, I wouldn't think you'd have to worry about the timing chain unless you were going to take the motor apart for another reason then it might as well be replaced.

Sounds like a good start to a project! Gotta love these old burbs