Welcome to potential Impreza ownership. Assume you will be looking at a wide track STi - in my opinion, with PPP, one of the best and frantic Imprezas built (I had a very late 56 plate a couple of years ago, but couldn't afford to run that and the current 2.5 WRX PPP!). Obviously, you will be looking for a full service history, confirmation of an HPI check or similar and a long test drive. On my STi, I fitted a Prodrive spring kit and Prodrive 18" 8J rims, which really improved the handling. Added a straight through centre pipe and a Pipercross foam panel filter. Also a Forge VTA (also on current 2.5 for 5 years, with no issues). Good to hear of another mature owner - I'm 64, wife 61, had Imprezas continuously since 1997 and both still loving it. Noisy, not too comfy, expensive to run, but positively addictive!!
Good luck and keep us informed.
John