honestly you could get stung either way....
1) you know the car and its history, mileage is really only material if there is rust or you have other expensive alfa issues to contend with such as body computer/ecu/wiring, that can get expensive. the only other things are as you well know on 180,000 miles the usual suspension stuff and cambelts.
so, are you on the original turbo? has the cambelt been done recently, are the wishbones due, any electrical gremlins..? if it were me I would calculate what its going to cost me in the next year vs the cost of buying a new car, the situation of course is that if you buy another lower mileage GT what guarantee do you have that you won't have to spend a shed load on that?
these cars anywhere upwards of 60k can get expensive as its wishbone/cambelt time.
those two items alone are £1000, throw in an EGR valve and another ''unexpected and you have spent half the likely purchase price again.
what you do have on your side is it seems that a 10 plate clover can be had these days for around 6-7k throw in yours as a part ex and you are shelling out 5k. ..that figure vs your maintenance calculation squared by your desire to have a new toy and you have your answer.
really nice to hear that at 180,000 the engine is still going strong! mine will turn over 100,000 this week and I would not change it, no way. its a fantastic car and I hope she will be with me for another 3 years at least.
this is all assuming you plan another GT, and why wouldn't you having got 180,000 out of the first