Re: GTV 3.0 V6 running costs?
I don't think a GTV will require any more maintenance than other cars of its type - they all have bad points - or design features depending on your viewpoint.
Any Cup will now be due a cam belt swap if it's not already done, and you'll need £300 or more for that depending on whether you use an indie or stealer. Obviously, then you're ok for 5 years based on your proposed mileage.
Front tyres wear quicker than many car because of the torque, inherent understeer, and nose heavy weight, and the rears can wear prematurely because of negative camber.
I get 17/18mpg on my daily commute and can reach 26ish on a run, as said above 21 is a good average. Insurance can be expensive unless you're over 50.
Mine's had a MAF and rear dampers (@26k). Consumables such as battery and exhaust are original - though I had a stainless steel one fitted to liberate some of the glorous noise. The wiper mechanism jumped apart last week, but that was cured with some grease and a press!
Look carefully for a good one, read the posts here, get it inspected if necessary and enjoy. I think they're great cars.