Auto residual value and reliability?

I can't say what older autos are like but I have a 11 plate A3 S-tronic (auto) and I will not go back to a manual. I am sure someone will be along soon to give you further advice...good luck and hope you find something.

If it has the DSG type gearbox I don't think you will find it's any less reliable than a manual gearbox. The DSG is not like a 'normal' automatic. It's basically a manual gearbox with two electronically controlled wet clutches running in an oil bath which Audi say should last the life of the car with the actual syncromesh hubs being moved using hydraulics. The only maintenance is an oil change every 40k. Also a DSG and it's clutches cannot be abused as the electronic will not allow it. If you attempt to over-rev the engine in any particular gear it will change up and there is no way you can 'slip' the clutch and lessen it's life.

I have owned three A3s with DSG gearboxes and done around 35k in my previous two and around 30k in my current one and have never had the slightest problem.

See this recent thread about the DSG for a lot more information and peoples thoughts:
http://www.audi-sport.net/vb/new-a3-s3-sportback-8p-chassis/124560-dsg-what-does-everyone-think.html