It really depends on what you want it for. If you want something easy to fix and can make some decent power, than you want an older 850. Most people just expect these things to go to 300k... they are very under built engines, meaning they are way stronger than they need to be for their factory output.
If you want to jump into the S40/60/80 go with the 2002-2004 Phase 2 styles. Tey had twin scroll turbos as opposed to single scroll and upgraded MAF's. The downside to these cars are that parts are hard to find... and when you do find them, you WILL pay for them. But they are "Full Race" quality, and make HUGE differences. My perticular car, is really quite amazing to me. I still cant get over my trip out here, and how after driving uphill almost the whole way, against the wind, and towing that trailer, that the motor didn't just explode from being in boost nearly all the time. But it STILL got 20mpg doing it.
Volvos are similar to M3's in that they aren't the absolute best at anything, they just do everything extremely well respectively. Mine is working its way to the 90k mark, and it still runs great... still gets 32-34mpg on the interstate and will shit and get from the traffic lights if i want it to.
From what i understand, this car had about 120k on the clock when it was put together, and makes more than 500hp without much problem. Its also using a V70R turbocharger, the TD04-19T... not a very big compressor to be honest.