First - Just get one bought - You won't regret it at all!!
Milo on here is just 25 and had an Impreza Sport Special and managed to buy a Legacy B4 RSK RFRB and it is cheaper to insure than the Sport.
You see the bigger Legacy does not gain the insurers boy racer status therefore the insurance will be cheaper.
I have a Forester S/Tb Jap Import - still has 250hp and cheaper insurance than an Impreza.
If you buy older than a 2000 you will escape the emmissions based road tax.
We pay £112 for six months tax for our 1998 Forester. Mind you Milo pays £210 odd for six months tax on the 2000 Legacy B4.
As for running costs - service it yourself - you will have fun doing it and plenty people on here to help you.
Fuel - Well if you want fuel economy just stick with your oil burner.
The Boxer Turbo is okay on fuel but if you have your lead boots on you will get into single figure mpg values - but - Wow, what a rush! - Sod the price of the fuel!
Again - Just buy one.
Oh - Buy the way - Welcome To NESF -