I've been on the lookout for one for a while. I've admittedly got a soft spot for the AD (I have an Inter-10 from 1980 that rides like a dream, barring a sticky jockey wheel on the Huret RD). If I were in your position, and if the bike fits, I'd see if the seller were willing to take ~$550 and go from there. It's not a steal, but nothing this nice usually is. All in all it's probably a fair, if a little inflated, price.
Here's one on ebay (something you don't usually see) with a starting bid of $600 and a reserve:
http://cgi.ebay.com/Classic-Austro-D...d=p3286.c0.m14