Background-
I bought a 02' WRX about 5 or 6 months ago and it ran great, i purchased it at 108k, and yea that is really high and i prolly shouldnt have bought it, the guy sold it for 10k and i was searching for a WRX for years, but near my area there are never any wrx's sold. anyways it has around 118k on it now, i have really been driving it and it still is in good condition no apparent dmg to the engine there is a CEL on though, tranny still grinds sometimes in 1st and 4th but thats normal. i am starting to upgrade the car and i am wondering what things i should be checking for, or even if i should be upgrading it that much since there are high miles on it and putting more performance parts on it will only make the engine give out sooner unless the right things are replaced. the reason why my CEL is on is beacuse of my TBE that i put on and a CAI, not enough Back pressure and MAF air sensor is reading too much air. i took it to a mechanic a couple times and he ran tests on it and thats what he told me, he said otherwise it ran great for what miles were on it. It has had regular checkups and services before and after i purchased it, i went ahead and put full synthetic oil in it and full synthetic for the tranny.
Here is what i have on it so far-
K&N CAI
RS-R* TBE
OEM sparkplugs (i originally had Iridium tip but that was making a CEL error, and the computer was reading a missfire in every cylinder, if that was the case then i would notice it but there was no apparent missfire at all, the CEL error went away after the OEM plugs were installed)
Boost,Air/Fuel,Oil Pressure Gagues (really soon ill finish hooking up the air/fuel and oil pressure regulator)

Inorder what im putting in next-
Turbo XS Utec (or after everything installed get a ECUtec reflash which is a lil cheaper)
Larger TMIC
Up-Pipe (or get headers with an up-pipe)
STI 565cc Injectors
VF22 maybe get it done by deadbolt (was really considering about Deadbolt TDO5 but wanted a lil cheaper factory turbo)
stage 1 or 2 Clutch Kit (dont know yet but so far stock is doing just fine will have to see when turbo is installed)
possibly some RA Gears (prolly a tranny rebuild)
Lightened Flywheel maybe around 9-11lbs

as far as engine wise that is what i was planning on doing sometime later. im sure that i am leaving something out but off the top of my head that is what i was thinking about. please if any other or better info tell me. thanks

 

if i were you id buy a new tranny since those where a big problem for the bugeyes

 

The first thing I would do is have a reputable shop or dealer perform all scheduled maintenance. Your car won't last too long after being modified if it isn't maintained. If the previous owner claims that all required maintenance was performed, I would ask for receipts.

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0==WW==0
"…axles of evil…" - george w. bush

 

Probably not a real good idea to be going crazy with boost and the like. The engine has a lot of miles on it, and you have no idea how the car was driven before you purchased it.

As was mentioned, have a reputable shop perform all scheduled maintenance. As for your transmission, just wait until it blows. There's no sense in spending money when you don't need to.