Downpipes are for turbo applications. If you don't have a turbo then don't buy a downpipe
. What do you have, ST or GT? Buy some cheap headers off eBay and reinforce the welds on them.
Anyways... There really aren't too many companies who make exhausts for the Celica. Greddy has the SP2 system, but other than that we really just have to buy a muffler we like and take it to an exhaust shop and have them fabricate the piping. You won't get the rumble a scooby has, simply because you don't have a boxer engine. HKS makes good mufflers, as goes Magnaflow. If you want some rumble, I would say magnaflow.
Shocks and Springs, if you aren't looking to do autocrossing or anything like that I would say KYB GR-2's. Good price and excellent ride. I am partial to coilovers, simply because I have them. NO EBAY BRAND COILOVERS!! I don't think suspension is something to skimp on. I have Ground Control coilovers and they ride like a dream. The KYB+GC's will set you back at least 500, and then you need to have them installed. A lot of people on here have TEIN S-tech springs, and not one complaint. They don't cost too much ($140-$160) and have wonderful ride quality.
Rims... its one's own decision. I have a set of Rays Engineering wheels and 7th Gen GT wheels. Both look awesome on the 6th gen. Also a good looking wheel is the 16" 7th Gen GT-S wheels.