Need Quattro Buying Advice

[Mark Plotz <mplotz@gac.edu>]

I am looking to buy an Audi Quattro and have been looking in the Upper
Midwest, where I live, for about 5 months. I am looking for a car that is
1986 or newer with a manual transmission. I have driven a 86 and an 88
5000cstq, both of which I really liked. I have also driven an 88 80 Quattro.
My priorities are: fuel economy, tightness (ie. structural integrity) and
good brakes (anti-lock preferred) Right now my buying options are:

1986 5000 Quattro. $4500 firm. 100K. Driven and serviced by an Audi
mechanic, and everything works-right now.

1987 4000 CS Quattro. $5900, can be talked down to $5000. 77K and driven by
a little old lady.

1988 80 Quattro. $5900, don't know if the price can be negotiated. 110k.

        The limit of my price range is $5900. I was impressed with the
tightness of the 80. I like the looks and space of the 5000. I have never
driven a 4000. I need a quattro soon, before it snows and the prices go up.
I don't want to spend all my money on a car and then find out that it is
going to need another $1000 in repairs. Which one will I be happiest in?
What are the differences in the quattro systems between the 4000/5000 and
the 80 (besides the differential switch); and is one system better than the
other? I will be taking advantage of them, so it is important. Best fuel
economy and best preformance? I am used to driving VW Jetta GLI 16V's and VW
Golf's. How do they compare? 
        Thank you for all your help!