There are several known, common leak areas for oil leaks, the oil trap breather box seal (about a 3.5 hour job, intake manifold must be removed) and turbo drain tube o-rings (about a 4.0 hour job to replace both, the trans is in the way making it hard to access).

I've also seen those radiators go bad.

As for the GM trans, it's a GM trans that Volvo "improved" but the first couple years of them (especially 99) were bad years.

Before paying a tranny shop to rebuild or replace it, either re-try your dealer and go on a test ride with a tech to demonstrate the problem(s) or try another dealer as that trans has known issues and any good dealer should know what to do. There are several TSBs on software upgrades and possibly replacing the STH solenoid (a 7 hour job) if the trans shudders in some situations.

What to do about this and the rest of the car? It won't get better with age and will be a "money pit" as more costly problems will arise. '99 was the worse year for S80s.