for the Technical Service Bulletins/Manuals. I downloaded all the relevant documents to my computer, as well as onto a USB stick I keep on the key chain withe Bentley keys. The TSD's are cumulative, don't restrict yourself to only 89 and up, some of the earlier material is still relevant to your car, particularly TSD 4400
for your battery issue, battery should not be discharged in a week or less from just sitting. Not that much electrical drain from the car. The stock alternator is marginal at best. Something like only 120 amps. The existing battery has to be near the end of its natural lifespan. Buy the best new battery you can find and stick the new battery in to ensure each time you start the car is not an uncertain adventure.
The Turbo R is just a car. Seems unnecessarily complicated to me in how the designers went about some things,and the electrics are primitive compared to current technology, but the only thing I have not been able to deal with on my 90 Turbo R myself has been the air conditioning as I do not have those tools. I have sorted all the electrical issues, replaced all four hydraulic accumulators, swapped out the engine mounts, replaced all the rubber hoses and lines, twin distributor caps and rotors, etc., etc. I still have a leak in the hydraulics at the reservoirs, just received all the necessary seals from IntroCar and am about to tackle that repair, once the car comes back from the air conditioning shop where it presently is for the third time this summer, bloody ****.
My observation, after only a year of ownership - the more the car is driven the better it runs. While I put the car away for our bitter winter, I am daily driving the car from roughly late April to likely mid October this year, weather dependent.
Parts are readily available. Even though I am in western Canada I have ordered most of what I have needed from IntroCar in the UK, what they could not supply I got from Flying Spares in the UK. Both have been excellent to deal with. Other stuff, like the big under car Bosch inline fuel filter and the Bosch fuel pump are readily available from most parts suppliers local to me, and I have had a really good set of spark plug wires made by Magnecor. https://www.magnecor.com/magnecor1/main.htm
Have fun with your car. After you put a new battery in it, then put a bunch of fuel injector cleaner in and think about taking the car out for the proverbial "Italian Tune Up", as many full throttle uphill runs as you can manage without getting yourself arrested. Warm the car up to normal operating temperature. Foot flat to the floor from just off idle to full revs......... repeatedly. Worked wonders for my car anyway. Your decision of course........ I anything is marginal on the car, these full throttle runs should be when it breaks, and if it breaks it needs to be fixed anyway, might as well get it over with, just my own opinion of course.
No idea about the excess moisture from the tail pipe on start up. my car does not do that. The only thing that comes to mind is a possible head gasket(s)/coolant leak, but if that were the case you would also expect to see increasing amounts of white smoke/steam from the exhaust as the car is driven.