i would say 20-25mpg is realistic from an STi
Of course that will go down when on those late night blasts
but hey.... who ever bought an Impreza for fuel economy
As for servicing i would budget something like this £120 for an oil/filer change (A service every 5-10k), £350 or so for your plugs, filters and oil/fluids (B service once a year/every 10k) and some thing like £500+ for the belts one which i think also covers diff and gear box oil (C service i think thats every 30k)
and i think the schedule goes something like A, B, C, B, A, C, A, B, C
all this was off the top of my head so not completely sure
Hope this helps