1. Springs, shocks, sway bars, bushings, wheels, tires, etc.
2. It really depends on what you want to do with the car. It can cost as much as you are willing to spend. I have put $4k in a total setup that I will put up against lightly modded C5 vettes (as far as corner ability).
3. Most of the modifications aren't very hard to install, but you will need to gather some knowledge on how to do them.
4. My car feels great as a daily driver....I don't feel that it is harsh or rough, but this is something that is specific to each individual person.
There is good info out there about suspension upgrades. I would suggest searching through all of the posts in the suspension section to see what questions you can answer for yourself and then go from there.