When I bought my 1998 S90 five yrs ago, I immediately ordered the Volvo factory manual of wiring diagrams. It has been a huge help and has paid for itself many times over in letting me figure out and solve some electrical problems. As you note, the 960 and S/V90 are MUCH more electrically complex than the 240. The factory wiring book is 250 pages.

My S90 has been quite reliable. A power steering pump, starter motor, rear shocks have been the only non-maintenance items replaced. Make sure the timing belt and associated parts are within their "best before" mileage, or replace!

I recently averaged 27.4MPG (US) on a very long trip at Interstate speeds. I use Mobil-1 5W30 synthetic oil, the Mann W917 oil filters and Mann air filter, NGK platinum plugs, and 36psi in the Michelin Pilots per Volvo's spec on the door sticker.

If you are not sure when the trans was last flushed, I would do that. (how-to details in the 700/900 FAQ's). Synthetic ATF is often recommended on this BB and it is great stuff, but I use Dexron VI, which is fully compatible with the Dex III that was used when the car was built, It is close to synthetics in performance, and is substantially cheaper.
--
Bob: Son's XC70, daughter's 940, my 83 240, 89 745 (V8) and S90. Also '77 MGB and some old motorcycles