Advice:
When one is upgrading a car, broadly one should consider the following in the order below, with rough cost-return;
(1) SAFETY (belts, bars, seats, hemlet). Common sense says start here. make sure you have these parts working properly.
(2) CONTACT (air, tires, wheels, lugs). Without good contact with the road, the other stuff doesn't matter. Light weight keeps better contact.
(3) BRAKES (fluid, pads, rotors, calipers). If you can't stop, then don't start.
(4) HANDLING (sways, shocks, springs, bumpstops, bushings). If you can't turn, then don't try.
(5) POWER. This is where it gets fun. There are only three things to power; fuel, air and spark.
(5a) Fuel (quality/octane, injectors, regulator/rail, pump)
(5b) Air (anything to increase intake volumetric flow and decrease temperature including exhaust)
(5c) Spark (plugs, wires, timing)
