I'd leave it stock for at least 6 months. Learn what the car can do
out of the box (pleanty, I own a 98 S70 T5M, too) before changing it.

Avoid anything that promises power for easy money. They don't really
work. ECU upgrade and exhaust are the proven methods for power.
I'd also make sure it is running as it should with proper mainteance
(plugs, air filter, synthetic oil, etc.) before anything else.

Suspension is a bit soft (drive an 850 and you will see why). But
I like it for my driving habbits. High performance tires would
probably make the biggest difference. Others also swear the
front suspension cross bar that ties together the strut towers
help (I can see how it would, the chassis is not particularly
stiff).

Finally, take it easy. One of the important things of driving is getting
the experience to learn what the other guy is going to do. No matter
how good your skills, it just takes time and experience behind the
wheel to figure out that someone signaling left does not mean they will
turn left, etc. Oh, and to everyone else, you are the other guy, too :)