Take it to one of the Subaru Specialist shops as listed on the Mechanical section sticky (called ECU and tuners)

Get them to check it.

I would do the following as a minimum
- compression test
- suspension play
- dump some gearbox oil out to check for metal
- check plugs
- check service records
- check oil consistency
- check noise in cabin for ticking, pinging, etc

Most workshops will do this and charge you probably something like 50-80 for it
I rather workshops than RAC, SGIO. Reason is a WRX is a little different to say a Barina, Commodore. At least a specialist will know what to look for in the "weak" points