These are experience:
first of all, compression test, have a few people bought used betty, only to found a blown piston ring not long after.

if result is ok, get air/oil seperator, best tbe u can afford, cai, a tune, and use 40+ weight oil,

then short shifter KIT, swaybars(front should be thicker), brake pads(project mu/endless, or evenfront rotor), better tyres(advan ad08, dirreza z2, pilot super sport), coilover(rear spring rate need to be 2kgmm less than front, hatch's rear is much lighter than front), rool centre kit, h-brace

finally wheel alignment.

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MY08 STI 422whp at 1.5bar on E85, built by Diversion Garage, assembled and tuned by EvoR

Update from new owner: 375whp at 1.4bar on 98, at Tokyo Motorsport