They are fantastic cars but good ones are hard to find and priced accordingly. A friend of mine bought one last year and we had to look around at quite a few before finding a decent one

Bearing in mind they only made 1000 finding a good one isn't easy
Most of them will have had a new engine or a rebuild by now, my friend decided to buy a tidy well looked after example that hadn't had a rebuild and budgeted to do that at a later date, only 2 months or so later he did a compression test after trying to sort out a poor idle and found little or no compression on one cylinder

That was caused by a bent valve though.
If you are still looking for one I would try and find one that has been looked after, good examples will hold their money all day long
The Prodrive map wasn't as good as it should have been so if you can it remapped for safety I would. A lot of imprezas also die from high speed runs because the air goes over the intercooler scoop instead of going through it

I've heard of lots of P1 owners exceeding 150mph in theirs and they simply don't like it

If you are planning on doing runs like that a front mount is highly recommended.
The Prodrive Alcon brake upgrade and 18"s are a good package so personally I would also look for one that has been fitted with those. The Subaru 4 pots aren't too bad but they aren't brilliant and for some reason the P1 came fitted with single pot rear calipers and not the twin pots

You can upgrade to twin pots quite easily though
P1's drive very very well, they have a very close ratio box with a longer 5th gear so they cruise really well

Imprezas are always a bit thirsty but if you are looking for an Impreza you aren't worried about fuel

You must run them on either vpower or tesco 99 only though because they can detonate on 95 ron and that sends shockwaves down the conrods to the big end shells and knocks them out