Parts To Replace

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So I recently purchased a GTI that had quite a few mods done to it. After digging I found out there is a Carbonio intake, a K&N air filter, an APR HPFP, full APR exhaust, an S3 Intercooler, and a K04 Turbo, with a DV valve. All with a APR Stage 2 performance tune. Well, the DV valve ended up going out, (assuming a seal or gasket was bad) but not knowing much of anything about the car, the dealership it was taken to decided to replace the entire turbo system back to stock. Naturally, they set the ECU back to factory, which cased one of my o2 sensors to go bad. (They replaced the downstream sensor, not both) After doing that my ECU started throwing the P0420 code. The dealership told me I needed to replace the cat and exhaust back to stock, but I wasn't going for that. After more research(and testing the cat myself) I determined I just needed an o2 spacer, until I get another tune back on the car. My question to anyone; is there any other things I should go about replacing to ensure the longevity of the car? I plan on replacing the cam follower, the ignition coils, the fuel injectors, the PCV valve, and air filter, but is there anything else I should replace aswell?