I owned a 2002 (bought it in 2009) "buyege" 2.0 and it had a few minor problems in the 3 years I owned it.
I fitted a cat back exhuast and remapped it. That didnt cause either of the two issues I had.
Faulty MAF sensor
Fuel Hose leak
No other issues in 3 years of ownership.
MPG was 20-22 typically.
Road Tax = £300ish
Paid £3000 for it, and part ex'd it against a newer car... see below.
I then part-ex'd it for a 2007 2.5l "hawkeye" I bought this car with 45k on the clock in 2011. Sold it last year with just under 60k on the clock. I fitted a full decat 3" bore exhaust system on the car and remapped shortly after buying it.
Didn't have a single issue with this car, not one fault in 4 years of ownership. I bought the car for £6500 and sold it for £6000 4 years later.

The car was kept immaculate and treated really well.
Standard exhaust - 24-28mpg
My exhaust - MPG 13-15
Road Toad - £495
Use only super unleaded on Subaru Impreza engines, ensure that all fluids are replaced regularly and you shouldn't have any real issues imo.
One thing to note my 2.5l engine would only get 13mpg - I drove from Stoke to Santa Pod last October in mine and did 60 all the way there (another scooby was following me and he had a LSD in his boot so we took it easy) and then coming back at the end of the day I drove 70 all the way back. It used a full tank of fuel pretty much, and averaged 13-14 mpg. This was purely down to my exhaust I had installed though.
So in a nut shell - 7 years of ownership of 2 scoobys and only issues was a fuel hose tear (£25 repair - but I did have to remove the manifold header - I assume a garage would have charged a fair whack for that) and the MAF sensor (£30 replacement - 2 screws to take old one of and fit new one - takes less than 2mins)
EDIT: OH and one caliper siezed on me, I striped it down and took it to a metal work shop who had to forge a new pin which cost me £10 - this happened when I swapped my brakes. I did install braided hoses and uprated discs and pads at the time - sadly I decided to sell her a month later cos I wanted the cash for a house deposit.
Fuel costs = LOTS THOUGH
When I bought my first scooby I was traveling 25 miles to work and 25 miles home. But I moved closer to my work shortly afterwards.
This was mine