My RMS needs doing too! It used to be a slow leak.
1. No special tools needed AFAIK. You can get the '90 factory service manual from cardiagn.com. The AT section has step-by-step instructions for dropping the tranny (AT-124), and the EM section has the same for doing the rear main (EM-20).
2. The gasket in the aftermarket kits goes between the retainer plate (which holds the seal) and the back of the engine block. Unless you're going to try and fish the seal out without removing the retainer, you'll need the gasket as well. I think I've got the Fel-Pro kit on the shelf for mine, don't remember--and of course I can't vouch for it as I haven't installed it yet.
3. I assume you'll need the gaskets for the exhaust crossover as well. You may need hardware or more depending on how much the exhaust fights you on its way out. These also tend to break exhaust studs (holding the manifold to the head), though I don't know that having the trans out would make these any easier to access. I would check breather hoses and exhaust hangers while you're under there. Might as well change the fluid in the transfer case too.
4. The trans cooler is a good idea. There are a few old threads on here with size and type suggestions. I don't actually know what mine's rated for, I got it secondhand and it's not marked. I installed mine in series with the existing cooler in the rad, and also added a Magnefine external filter while I was at it.
If you just drain and fill, you don't get the old fluid out of the torque converter, so I did a flush through the cooler lines instead. Take the lines off the existing cooler, put them in a bucket, run the engine until fluid stops coming out, add that much fresh fluid to the transmission, do it again, repeat until what comes out looks more or less like what goes in.
The trans fluid level is a bit of a pain to set. It has to be warmed up when you check it. If you set it cold, it'll be too high once it warms up. Either measure what you take out and what you put in and make sure they're the same, or undershoot a little so you have room to top it up once it's warm and you can check it properly. I thought mine was underfilled, warmed it up, and had to drain a bit back out of it.
The manual calls for Nissan-Matic D, which is still available. From what I've read, it's basically just Dexron III, so I used generic parts-store-brand Dex-Merc, which was cheap and available locally.
5. Torsion bars are also a pain to set up. If you have to drop the bars (and the service manual suggests you will), mark, measure, or at least take a picture of where the adjusters are set and how the bars are clocked vs the adjusters. Otherwise you'll be screwing around for a while trying to get your ride height back where it was. I used spray paint to mark the adjusters on a friend's S10 Blazer when we swapped the trans, and reassembly was as simple as matching up the paint marks.
This thread recommends filling the gap between the RMS retainer and the pan with silicone to stop mud from getting in there and causing corrosion. Seems like a good idea.
Good luck!