Hi David,
I don't have all the answers to all your questions but I'll try and help out as much as I can.
1) Not sure unfortunately.
2) As far as I know the only way to add DAB to an RNS-510 is to swap out the headunit for a DAB RNS-510 and add the appropriate aerial. Happy to be corrected on this though.
3) Good, just go into any potential purchase with your eyes open (as it seems you are). Usual advice applies, ensure car has updated tensioner/budget for replacement.
4) Yes DCC dampers will cost more to replace than standard ones. Having said that I don't know of any long term problems with them. My car has DCC and I'm on 91,000 miles now with no issues there. I'll try log in to ETKA later and get some example prices.
5) None that I know of that are 2009 specific. Diamond cut Monzas can suffer from whiteworm corrosion. Problems I've had with my car during ownership have been failure of the coolant level sensor (expansion tank and sensor replaced under warranty) and failure of the PCV valve (DIY replaced this, ~£100 in parts from VW dealer). Been utterly reliable other than that. I think other potential weak areas include coilpacks (standard VAG) and diverter valves (mainly on tuned cars).
Would you be looking at manual or DSG cars? If DSG make sure it has been serviced on schedule (oil and filter change every 40,000 miles).
Hope that helps out,
Dan