It has 103,000 miles and a clean carfax report. Is there anything that should be known about this model or the five cylinder motor? I don't know too much about these at all. Any help would be appreciated. Thanks.

Run a search of the forums. That will turn things up. I just did a quick search and came up with quite a few hits.
I just got an 87 5000 CS Turbo Quattro 2 weeks ago with the same mileage. Sweet car. How much $$ is your friend looking at spending?

See sample below.

1) Leaking Power Steering Rack
2) Oil Leaks in general
3) Failed Motor/Transmission mounts due to oil leaks
4) Cracked or busted intake hoses.
5) Oscillating idle (due to bad intake hoses, idle stabilization valve and/or hard fuel injector casings)
6) Busted window regulators/sunroof
7) Bad climate control computer/control box (if bad the vents won't open, etc)
8) Bad brake accumulator (aka The Bomb) (brake suddenly and dash brake light will flash)
9) Bad electric radiator fan (idle car, make sure fan comes on)
10) Bad CV-Joints (drive car, on turns listen for clanking noise from front end).
11) Torn CV boots (if torn, likely CV joints will go bad soon)
12) Radio will go out often (can get rebuilt one from dealer pretty cheap)
13) In manual transmission cars, make sure 3rd gear synchronizer is good (down-shift from 4th at high RPM), I had two go bad on my old 84 5000S.

It has a recently rebuilt transmission.

Make sure the car is cold when you get there. See how it starts up cold. Drive it. See how it starts warm/hot. Listen for lifter noise. If you get some, don't be alarmed, but make sure it clears up.
Check all the power windows and sunroof for operational status.
Open and close ALL doors a few times.
Check for uneven tire wear.
Good luck