Wotcha and welcome Ben - We have a few fozzie owners on the forum with vehicles in various states of tune, now I have never owned a Fozzie but being on this forum a bit you pick up the odd nugget of information. As I understand it the Fozzie and the Impreza share the same basic layout which is supported by the fact you can get STi versions of both so in theory if you were to invest the funds you could have a 500bhp closed deck block 2 bar of boost running hay bale carrying workhorse of snarling death.
Check out Farmerpugs thread he took a similar approach to what I believe you are looking at - a sub grand workhorse, but at that price be prepared to put some wrench time in.
Fuel economy - its all a trade off, if you go NA then it will be less power but also less fuel consumption, the turbo versions use more fuel but dont feel as laboured under load - if you wear a diving boot when you drive the mpg is obviously going to be lower but the smile will be bigger - its all relative
Im sure Stants and the other Fozzie owners will be along in a while to contribute but feel free to get stuck in with the forum