Well where do you begin with a question like that...
The best thing you can do is get someone who is a mechanic / has a good understanding to with you when you look at the car. There are allot of good folk on here who would be willing to go check out an ae86 with you. Especially if they get to test drive it.
I'll break down what i look for into 2 categories
Mechanical
The obvious ones here are
* does the diff / gearbox whine, is there any unusual noises from the motor.
* When the front is jacked up, any play in the steering (wobble the wheel left to right 3:00 - 9:00) and up down and on 45 degree angles (it shouldn't have any play)
* Are the bushes cracked / torn
* Clutch does it slip , noises with the clutch in that go away when u let it out / vice versa.
* Engine mounts in good condition ( hold foot on break, and let the clutch out and push it back in just before it stalls) engine shouldn't rock too excessively
* Ideally a compression / leak down test would indicate the health of the motor, but not every one will let you do that.
* Oil leaks shouldn't be seen.
* A good engine conversion will look factory, if you see wires going everywhere or things that look out of place i'd pass on the car.
Body
Obvious ones are rust holes, people can bog it up to make it look good and give a quick respray. Rust in the sprinters is most commonly found
* EVERYWHERE, around windscreen, below tail lights, spair wheel well, roof above doors, firewall, b pillar ( where door shuts) and around the rear 1/4 glass and hatch
Placing a magnet on the car is a good way to detect bog.
* Bubbles in paint is rust
* Open and shut the doors, they should close easily without having to slam them
Look for the gap between the doors / guards, it should be even. Same with the bonnet / boot.
Hope that gives you a starting point.