the only free mod that would be worth a damn is the airbox mod, just cut the bottom half of your airbox off, leaving an inch or two of material for structural purposes holding the filter in.
the TPS, IAT and diffuser mods are thoroughly worthless.
a cheap one that will do you good would be the e-fan conversion, usually you can find a taurus or a olds cutlass e-fan for like 15 bucks at a u-pull-it, then you need wiring, a fuse, a relay, and a temperature control, probably another 50 bucks at most in parts.
A good rule of thumb, is money = power, the direct answer to "how fast do you wanna go" is "how much are you willing to spend"
to me free mods dont give any measurable performance gain, and will decrease the reliability of your vehicle.