New to this, need some help

Depending upon where you live will make the difference. Honestly, the price sounds high to me for the mileage. I have seen '98 S70 T-5 with 40k miles go for $15500 in the DC metro area. If you know the car's history and the owner, and trust them both, then go for it. If not, shop around. There are some great deals out there.

Prospero

BTW, mileage is not too important if the car was maintained and driven nicely. My 850 turbo has 183000 miles.

I see 850s going in the high teens, but generally they are low mileage or they are Rs. If the thing is loaded, or it's a special edition like that silver/white wagon thing then maybe, but the mileage on that doesn't sound all that awesome. It's about average I guess.

Consider checking Edmunds.com or KelleyBluebook.com and checking pricing there.

Are there any performance mods, i can do to it, i already know about suspension but, are there any mods i can do to make it faster?

Usually price is a function of how the market for Volvo is in your area. If the car has been maintained well sounds reasonable. Austin,Tx has many Volvo drivers this adds to the price. Other factors would include pre-select warrenty, etc. Wont be a pre-select option for this one. Too many miles.
So, again level of service is important. Have it looked at by a Volvo tech. who is anal about Volvo's. The '97 850 GLT I have currently is my third Volvo. I have two places I trust, neither are dealerships, with my car. Having them check over the cars before purchase has saved me money.