It definately depens on what class you plan on racing in. Some of the more stock classes won't aloow spring upgrades. You can upgrade as much as you want in street modified class. Just remember the cars get faster in the more modified classes, so choose wisely. For instance, If did springs first without any other mods, you would be at a large disadvantage to others who have more mods as well as springs in the more advanced class. As far as what set ups are good. There are so many parts available. My roomate and I are both auto x ing our CA18det powered 240's this year. We both have the tein HE coilovers. They are stock set up for drifting, but we set everything a lot less stiff for the auto X. They run around 1300 dollars, but they allow you to adjust camber, spring rate, damper, rideheight. Fully adjustable true coilovers. A crowd Fav at auto x's are the eibach spring/ adjustable strut combo. A bit cheaper but not adjustable. Definately to ones advantage if they don't know or want to spend time on suspesion tuning. It takes lots of paitence to achieve the perfect settings on a fully adjustable set up. LSD's are nice too. Finally IMO, the most important things that are overlooked are the rear sway bar, and a full bushing kit for the car. Both are cheap, although the bushings can be a long pain in the a**. Whiteline makes a great swaybar.
http://www.mofugas.com carry's whiteline, as well as other suspesion companies.
http://www.forcefedindustries.com is our shop in columbus, we can also order you what ever you please, except we don't have online ordering, so long distant. sorry. good luck