Congrats on the purchase!
2. I understand your concern here, Alfa petrol engines are design to use some oil and the JTS is no exception, even though I have a diesel I check my oil weekly to be sure. The timing belts should be changed every 36k or three years whichever is sooner and regular oil changes will help preserve the life of your engine, every 5-6k for me. Depending where you are in the country you could take your GT along to Autolusso, as they offer free surgeries on Saturday mornings, just make sure you book an appointment. Autolusso - Saturday Surgery
. Sorry I can't give you any more but my knowledge of the petrol engines is very patchy.
3. This is a common issue and I too am having issues with my brake switch, if it's malfunctioning it will knock the cruise control off but the part is quite cheap (£15 ish) so you might want to swap it out. If you move the drivers seat all the way back you can see the brake switch in its mounting above the brake pedal, it's a simple remove and replace job. (www.shop4parts.co.uk
4. Not sure what they meant by this, Alfa 147's, GT's and 156's share the same platform and I believe all were factory fitted with alarms and immobilisers. When you lock the car with the fob you'll hear one long beep as all the indicators flash and the locking mechanisms do their thing, two beeps when you unlock the car. You can see if it works by locking the car with the fob, then using the ignition key you can manually unlock the drivers door as it has a keyhole, the alarm should sound. Quickly unlock the car using the fob button so you don't deafen yourself. Please note that locking the car with just the ignition key doesn't activate the alarm, you will still need to use the fob button for this.
6. As mentioned you'll want to check the oil regularly along with other fluids, power steering etc, screenwash is located cunningly under the cars left wing as you stand in front of it and you WILL need a funnel. Very annoying
Listen out for squeaky suspension, Alfaworkshop have a great guide on the things to look out for and are parts suppliers also:- http://www.alfaworkshop.co.uk/alfa_gt_guide.shtml
The GT suffers from window demons from time to time but a quick search of this forum will show you how to correct the problem. The exhaust back-boxes are known to rust quite badly (easy-ish fix) and the GT, like the 147 and 156 sometimes enjoy eating their front tyres. Specifically, if the inner edges of the front wheels wear heavilt this could indicative of worn suspension components or poor alignment, I take my 147 in every year before its MOT for an alignment. Also check the floor-pans underneath the car, some were poorly undersealed and can rust through but if caught early you can get this cleaned up and re-sealed quite cheaply.
Alfas have a loyal and dedicated following in the UK and this is reflected in the specialists around the country. Autolusso are the largest independent Alfa specialists in the country and really know their stuff, they have a massive parts department and perform everything from servicing to engine rebuilds and cosmetics. Autolusso
Alfaworkshop are also parts suppliers and perform all manner of service work. http://www.alfaworkshop.co.uk/
After that OpieOils will sort you out with service kits if you prefer to buy your own: - Engine Oil, Motor Oil and Car Engine Oils from the largest UK independent online supplier of Performance Oils
There are many more but these are the specialists I have experience with and trust me you won't be disappointed. Congrats again on the purchase and enjoy your GT.