The Db9 is a well rounded car. As far as service costs its not to bad as long as you keep up with the car. The V12 engine has power, as well as looks i own a service shop in costa mesa, i use to work at a dealer as a tech. Feel free to conact me for more info.
It should be one of the most reliable options out there...if you will be very attentive to the oil system.
Otherwise go for the manual Coupe, the TouchTronic is a fine transmission, but is a compromise. There has been a small issue with the automatic version "park regime", but I think it's been dealt with.
Older cars should be strong so long the oil system is maintained.
I would avoid the Volante version at all cost.
Lastly, waiting for 3 years is the best option for you...
Once they get the DB9 replacement into production, prices will bottom (possibly like the DB7's). I doubt the cosmetic makeover due in 2011 will pull the prices down, so consider waiting a bit.