Listen bud, its a bit late to tell you that you should have checked this car over thoroughly, or got a Subaru enthusiast to look for the obvious things. You probably dont want to hear it so I think you should look at the car on the strength of how much its going to cost to repair. As stated above, first port of call, headgaskets. It looks very much like you have the full dead headgasket symptoms, and if it wasnt that way when you bought it, running it to full temp will have killed it. You are probably looking at around £800 to get it sorted, but dont scrimp on the repair or you will have compression problems etc. Ie. dont trust some backstreet garage to do it, get an expert to do it for a bit more cash.
Dont use the aircon until a mech can have a look, could be a simple fix.
Brakes sounds like a pad issue, but check the discs. They may be scored to hell, and be an MOT fail next time round. They dont have to be expensive.
Turbo oil drip. One for a mech to have a look at. Could possibly be dealt with when the engine is out for the headgaskets.
Headlamps. Get a mech to see where the problem is. Youll be in the garage already to sort all the above out.
Dont throw money at the coolant system buying new rads etc. Get an expert to look at it, do a sniffer test which detects exhaust fumes in the coolant system, do all the other checks that will prove its a headgasket. (I hope its not, but forewarned is forearmed)
Please dont think the Impreza is an unreliable car, it appears the last owner was the Josef Mengele of the automotive ownership game.