1991 Audi 200T Auto-tranny Qs

[L DC]

Hi all:
 
I came across a 1991 Audi 200 Turbo with automatic tranny for sale (slush-box I know) with 108K miles and have a few questions.
 
The car is basically being sold as a parts car because of the following:
 
1. Has/had a "small" battery drain and was in good running condition.
 
2. Backyard mechanic who proclaimed himself as an expert on older Audis took the whole dashboard and underdash apart, center console, some doors' cards and split with the battery leaving the car in that condition. Instrument cluster was left attached.
 
3. Car has been sitting close to a year under the elements.
 
4. Black sport leather seats  torn and dried out. But the electrical works on them.
 
5. Missing rear seat bottom.
 
6. Missing front grilled.
 
7 Tires are shot from sitting.
 
Prior to seen the car, the seller (small used car dealer) said he is basically selling the car as a parts/junk car for little more than $400.
 
I popped the hood and the engine is very clean: no rust, no oil leaks and new-looking brake master cylinder and NEW OEM harmonic balancer.  
 
I requested to borrow a battery to see if the car ran and the seller said: "I don't want to F$%^&&* with it. If it starts is going to cost you more if you want to buy it. If you take it as is a little more than $400 bucks."
 
I insisted in borrowing a battery. I unplugged the coil-wire and pulled the fuel pump relay out and crank the engine to build some oil pressure. I replaced the coil-wire and relay and attempted to start the car and on the second try it fired up.
 
Engine ran nice and smooth and idled just as good. Exhaust gases clean with no unusual smells; i oil burning.
 
Seller was surprised that car started after sitting for such long, but reiterated car ran fine prior to mechanic dismattling interior and said the price was now $600 because engine still runs. He also claims AC worked fine but could not test it because center console was taken apart.
 
I did not get to drive car because of the condition of the tires, which included one with thread coming apart.
 
I did shift into reverse and every single gear and the car moved, and brakes worked fine in the 5-10 foot drive on straight line.
 
Steering is nice and smooth and there were no weird noise coming from either engine or tranny.
 
There is absolutely no rust what-so ever in the car (probably a FL car all its life) and the BBS-like rims are in great shape as is the body, which is free of dents and/or dings. The car, originally pearl white, however, was re-sprayed at some point with a beige color that looks rather flat.
 
Anyway, on to the questions:
 
Is $600 a fair price, base on the above description? If I buy it, I'm going to try negotiate a bit down although seller is pretty firm on price.
 
What kind of  longevity can one expect from the auto-tranny and they really a piece of junk?
 
What kind of maintenance is required to make tranny last a bit more?
 
Any other things/issues to consider?
 
I figured if the tranny goes or is bad, I could yank engine out, convert it to EFI and stick it in my QSW.
 
Your insight is greatly appreciated as always.
 
Thanks in advance.
 
Best regards,
 
-Louis