I don't think there is the perfect solution for this scenario. Totally get that you want to reduce the amount of old Volvos. I sold off my s70 that I dearly miss, but it was just too much effort for me together with the T5-R. 2 old cars going on Oldtimers just isn't great. And it really had become s...

AC on my 850 is similar, slow leak, worn clutch, and not working great. I did take a peek at the evap, and it looks like something out of a horror movie. So needs to go, condenser also needs replacement, looks very worn. Need to replace the clutch at least. Dryer is rusted badly. Lots of work to do....

I'd also say it is not worth the effort and money. Agree on not getting used ones.
Normal shocks are absolutely fine, and for sure better than old&used nivomats.

No idea on the differences, mine were blue (also FWD) but no markings on them. Threw them away, were heavily rusted and beyond saving.

There are 2 main issues imo, legal (what modifications are allowed etc) and the technical side... not really worth it. Had the B5202S in an s70, so I feel you. Maybe add a short shifter if its manual, that at least makes it a bit more fun. Also metal bushings for the shift cables can help mashing ge...

You can of course now buy cheap brake fluid testers, but I don't know how accurate they are. Come to think of it I may have one! If I have and can find it I will try and see what it tells me. I use one of these cheap testers for a few years now, I think it works good enough to tell when your fluid ...

I'd replace it, just because they also age. Bad o2 sensor on mine caused massive fuel consumption and bad emissions, but I don't remember driveability issues. Might be something else, I'd still get a new sensor. Don't think a catback alone (without downpipe) would cause issues. Don't know about the ...

Regarding the transmission, if the fluid change doesn't help and it shifts weird/hard, you can also try to replace the TCU with a another used one. Mine had failed and it was still working, but acting strange for a long time, before it finally caused a code.