I traded-in my 94 non-turbo 850 for a 1999 S-70 GLT and here are my thoughts:

The 850 feels more solid-the S70 seems to have more squeaks and the front door leaks wind on occassion. The seats also squeaks.

The S-70 is a much faster car BUT it does not handle as well as a 850 (the ride is softer)-my 850 was a non-turbo. You must get the turbo (All the mechanics I have spoken with claims that the Volvo turbos are fine).

I do not know why Volvo changed the rear pass-through on the S-70 to a removable piece (the 850 had a flip-down); That is the most irritating thing about the S-70.

The S-70 feels more luxourious.

The S-70 is no head turner since many, many people did not even know I changed vehicles. Yes, I like silver cars!

However, I am still glad I made the change since the S-70 is a much better vehicle and my wife did not have to 're-learn' the car.

I would go for the 1999 since it has the side markers and I have heard that there were some issues with the 1998s. There is no real difference with the 1999 and 2000 models.

I would look for regular oil changes (preferably synthetic) and other typical maintenance items. Best year to buy is the newest one you can afford, with maintenance records and low miles. Why are you moving from the '96? It would seem that you will pay a pretty steep price to get a 2-4 year newer car with a little more power and have to feed premium fuel. Mind you, I have looked at doing the exact same thing but resale values for used cars are pretty poor. It's a different story if you are adding a car and not replacing one. If that's the case, skip the last few sentences.

The turbos are not really an issue. Transmission and air conditioning
are the big items. Check to see if transmission fluid has been changed
around 30K. Volvo says don't, but it is a good idea. A common problem
with transmission is called the PNP switch and costs about $300 to fix.

Air conditioning is a bigger issue. Many cars leak refrigerant. Fixing
costs well over $1000. A crafty re-seller will recharge the AC so
it is working but a slow leak would not be uncommon. Recharging is
maybe $150 and gets you a year or two; but eventually they need servicing.

Just noticed you already drive an 850, so maybe you are familier with
these items. We own a 95 850 and a 98 S70 and overall, they are similar
in reliability. I would not prefer a 98 over 99 or 00 but many people
will avoid a 98 since it is first year (might get a beteer price on 98s).

This site has many common problems and their solution:

http://volvospeed.com/maintence.htm