my honest opinion it would be to not mod anything. a turbo'd car that old would scare me! ive worked on a 04 with only 58k and the amount of oil built up in the TMIC was horrible. i personally would take almost everything off the motor and clean the HELL out of it! engine bay, hoses, tubes, bolt on parts, nuts, bolts etc!! (freebie)
then i would do an oil change ($30-40). at 3000 miles i would do another and take a sample of oil for an analysis ($25). after i got the report back id make sure i have a happy and clean motor. then id get an Air/Oil separator or catch can to keep my parts remaining clean! ($200-300).
then id pull the spark plugs and do a compression test to make sure my piston rings are happy and that the motor can take some upgrades. a leak down test could also be done. id also pressure test the inlet and turbo and manifold etc with a home made tester. ($10)
after all that id start with the basics, tune first and foremost as the stock tune kind of sucks.
Scott-2011 WRX 335HP 394TQ 93 VF52!