Although, I believe that the 997 is a better car than a comparable 996, I would very strongly consider a new 996tt if the price was right. The n/a 997 has no where near the torque or midrange (usable) power of the 996tt. I didn't take the time to convert your posted euro $, but a discount of 18-20% would make me buy the tt.
Although the new tt will make the 996tt prices drop, much, of that has, most-likey, already occurred.
I own a 2005 997S and absolutely love it, but these cars have depreciated a fair amount already.
I would check a major dealer market, like Manheim (USA) to determine what the used 996tt's were bringing at the auction. This will give you a reference point. Also consider that you will have a longer warranty (transferable) and far less miles than a used tt selling at the auction, so add a few dollars to what you would pay for the 996tt from the dealer.