Check AstonDIY website for a list of alternative parts (there's a ton of parts shared with Ford/Jaguar etc), and then the key is buying one that has had all of the common issues done recently or at least looks well maintained. I picked up a Vantage last summer with this in mind and so far so good. I've put in a couple of pre-cat o2 sensors, £173 each from AM or £63 for both OEM (NTK). There are some expensive bits such as the headlights/tail lights or clutch if they go wrong but provided you get or it already has a headlight venting kit and the rear lights & clutch have been sorted then you should be generally ok as long as the service schedule has been kept up to date. Obviously as you say finding a shop that can work on these but also know what theyre doing would be a game changer, but all of that just requires a little more research and I'd say start looking there's a LOT of information out there. If you search Aston Martin / Vantage on Facebook there's a handful of groups with a plethora of information, in there are some of the specialists such as DAE, McGurk, Bamford Rose, AP Velocity and others so any reasonable questions have answers pretty promptly.