the aftermarket for the car has definitely dried up, but there is still stuff out there if you don't mind spending some money. sure there are more accessible cars to make power from, but this car is pretty unique and attracts attention where people almost roll their eyes seeing another subi.
Pope makes most of the bolt ons you would want to make that kinda power. he has an smic that is bigger than many fmics, upgraded cooling system, upgraded stock replacement manifold, 3" downpipe and exhaust and sell's the unichip tuning system that works well with our cars.
expect to spend as much as the car is worth though. if you bought all the pope stuff that would be like $3000 but its all bolt on and proven effective. you can dick around with some ebay stuff but i wouldn't recommend it.
250 would be the upper limit of the stock engine and if your the type who beats on the car all the time, it will not last. you will want at least a set of k1 rods, though i don't think you really need to go through all the build effort many do if you have a good block to start from.
as for milage, my msp has 147km on it and i got it new and daily drive it, so not all cars are high milage for the years.. just treat it right and it will be reliable.
i wouldnt consider modding the car though unless your doing all the work yourself. if you have to pay a mechanic to do it, you will quickly overshadow your cars value with even simple mods...