Basically the Ad is saying, that there is a 2000 si, which is modified, so it's prob. had the hell driven out of it, so just like any other car, check for wear on parts, funny noises, grinds in the tranny, any smoking ect.
As for the mods, it has a good catback exhaust, which will not really give you anything power wise, but should sound decent.
A ho-hum suspension setup, I have not heard many good things about Skunk2 coil over kits personally, but thats just me. Coil overs will make the car handle better, but the car will be lowered, so make sure you don't hit any bumps and the ride quality will be kinda shitty.
The grill and headlights are cosmetic, up to you what you think of them.
Rims could be nice too, and the tire size is pretty good for that kind of wheel, but he makes no mention of what tires they are, so hopefully they are good ones?
Any who, I would suggest from now on, you use the SEARCH button in the upper left hand conner of your screen on SHO. You need to learn a lot about your car before you do anything. I am talking first off price, and then if you get the car, things about it, such as what it can do, what motor it has, how to take care of the car and it's motor and tranny and so forth.
I'll leave the rest of the research and stuff to you. Just check for trouble spots on the car, just like any other car, and don't pay to much and you should be fine, I would be able to give u some sort of idea on the cars value, but IDK what an Si goes for. I'm a teg guy, Integra > civic
Anywho, good luck bro