Generally affording it isn't an issue. Unfortunatly (or, if you're buying one, fortunatly) they really don't hold their value any differently than any other V6 6000. The only case you'd get into is if someone actually knew what they had, which with an A6K is rare. Most people just assume it's any old box-shaped late 80s GM sedan. They really are incredible cars, and the only reason I don't drive mine that much is a) two cars are enough for me to keep insurance/licenses on and b) most of my driving is done between school and home now, and the 14 MPG (ouch) the 6K gets just isn't practical.
Yes the gas mileage is THAT bad. The 6K SE/STEs were porkers to begin with, let alone with the added weight/driveline loss of the AWD.