Depends, what make is your boost gauge ? You could buy two more of the same brand and save some money, or buy 3 new ones, Various places like scooby world do kits for 200 I think, how handy are you with electrical stuff ? Some of them daisy chain into each other so you only have to wire one into the car and that powers the other two, some need all three wiring separately. It's fairly easy but fiddly as depending on where you locate them access can be a ball ache Has it been mapped ? If so ask the previous owner which fuel it was done on, jap cars run on higher octane than here so vpower is the closest we get here.
Don't bother buying cheap oil filters genuine subaru ones are about 9 quid frpm dealer maybe cheaper on line.
Also worth checking if it's been undersealed and when, they don't salt the roads in japan so it's not as well protected underneath
Diy kits are between 10 quid for a pot of the brush on stufg to 50quid for a big tub and spraying applicator. It's messy but well worth getting underneath and the wheel arches coated before winter if it's not been done for a while