Recently bought a 2004 Celica GTS (from what I understand).
I plan to tune it up for power and handling gains for a weekend track car and in the future focus on cosmetic upgrades.
I have a few questions...

1) I'm looking for coilovers that will stiffen up the suspension and increase handling ability?
2) What mods can I do to increase speed, power, etc? and what brands should I stick to? (Naturally aspirated as I have heard it is not possible to turbo due to lack of space)
3) Can I retune the stock ECU? If not is it possible to replace it?
4) What wheel size, width, and offset will work on my car?

Any help will be greatly appreciated! I'm a qualified diesel mechanic by trade so all upgrades will be done by myself, this is my first project build and I want to avoid wasting money on things that are not needed!