Intake and exhaust are mainly up to you since well exhaust is pretty obvious its what your going to hear a lot anyway so for cat-backs or axle back its up to you, I personally like my muffler delete, sounds great but prepare for some ear rape on the highway with the 5 speed. Though if your looking for headers I'm not sure, I got myself some Megan Racing headers but haven't put them on the car yet, I was gonna dyno it before I put them on and after to see if it does something, they seem good quality though and I feel the suction form them when I blow in one tube, but I can only assume. Intake is up to you really on how much you wanna pay I have a K&N but its in the most useless position ever (previous owner put it behind the rad...guh) its about air passage mainly, picking where the best place to get air from in a Celica is very limiting. Suspension, personally I would do air-ride, and if your concerned about performance on air-ride go watch gears & gasoline's vid on that its better than you think. The only turn off with air-ride is the expense, but at least you can go as low as you want and not be scared of speed bumps. On the cheaper side though I hear K-sports coilovers are good, quote me loosely on that, someone with experience with them may say otherwise.
Hope this helps somewhat.
