really tough to say without seeing the car. since you were talking about free mods, i think BG meant to say removing the airbox "silencer", not whole airbox, as a CAI costs money. and the
K&N is worth the money, esp if you need a new air filter anyhow (unless you want a CAI sometime in the future).
for now, i would do as BG said, and just give it a good tune up. generally i spend a hundred or two (or more) doing this. change all fluids (oil, coolant, trans, rear end, etc, and all filters (oil, fuel, pcv valve, grommet, screen, and so on). do plugs, cap, rotor, serp belt, etc. i would do this and drive it for awhile to worm out any problems. nothing like doin a ton of work and finding something not working and havin a million variables (when it comes to mods, i try to only do one thing at a time). just doing all this will likely boost performance, as it has probably not been done lately.
good luck with the new purchase.