Wow, 100k miles on an 04' model. That's nearly 100 miles per day! The previous owner must have been a Taxi driver or something.
As for performance, it depends on your budget:
FREE! - oh you know its got to be good if its free. The simpliest and easiest way to make your car faster is to reduce weight. You can start with whatever clutter sits around. Try removing that spare tire and jack (it weighs about 35-40 lbs). Drive on 1/2 tank of gas or less (this will drop you about 40 lbs). During the dry summers, remove that windshield wash which you don't use (about 10 lbs).
Less than $500 - these are weight drop methods which will cost you. Install a lightweight battery (about $100). This will drop you about 40-45 lbs. Get some lightweight rims ($50-$100 each). This can drop you up to 40 lbs.
Less than $1000 - exhaust and intake are usually the first steps for aftermarket mods. Free-flow mufflers can cost about $100-200 depending on quality. A complete cat-back system can run up to $500 or more from reputable manufacturers like Magnaflow. CAI systems can be very cheap or very expensive, depending on who you buy them from. Their gains are neglegible. Along with changing the cat-back system, you could change your exhaust manifold (header). Those parts usually run around $200-400. In all, redoing your exhaust and intake can push your engine up about 5-10%.
$1000 to $4000 - a nice supercharger or turbocharger kit would work in this price range. If you install it yourself, it would be much cheaper, but to pay for someone else means you better be ready to fork over some serious money. Alot of fabrication is involved with turbochargers. Superchargers are easier to work with which is why TRD uses them. The gains from a turbo/super kit can run from 20-40% gains, depending on how high you set the pressure to (be warned, the higher psi you tune it, the faster it will wear down your engine).
$4000 and up - for the extreme spenders, engine swapping is the best option. You could swap in the Corolla XRS engine (170 HP), or the Celica GT-S engine (180 HP), without requiring huge amounts of fabrication. If you do the swap yourself, it could cost $3500-4000 depending on how good an engine you pick up. If you pay someone else to install it, add about $2000 onto that price tag.
