Take a good mechanic with you.
Car should be checked during the daylight. Ask the owner to have available any manuals, service, and maintenance records, as well as title and VIN number (you can get a VIN report on Carfax.com).
Things to check out:
1) Chassis (look for bends, repairs). Look at where the frame connects to the bumper with factory welds or bolts. If the frame welds and metal pieces are identical and straight on both sides and no parts of the frame have been cut and welded back together, then you can safely assume there has been no frame damage.
The radiator support is the bar that runs along the front of the car and is attached to both fenders and supports the radiator. Look to see that it is straight, shows no signs of being bent, smashed in or repaired. Check the bolts and welds.
2) Transmission, repairs are costly make sure it shifts right, if its an auto
tranny check the smell of the fluid, burnt smell could mean worn internals.
3) Body: look for repainted parts, look for paint cracks, imperfections.
-- a) Damage to the paint: if its local to a fender, bumper, etc, substract $300 ea, if it needs a total repaint substract at least $1,000.
-- b) add 20% above that if the car needs bodywork ($360 for panel repair, $1200 for entire body repair)
4) Check the engine oil, if its milky, most probably water is getting into the engine, say bye to the seller.
5) Take it for a test drive
-- a) Should drive straight
-- b) All gauges and electronics including the A/C should work
-- c) turn on the A/C and check the engine is not overheating
6) Check
http://kbb.com/ for the cars original value.
VERY IMPORTANT: The seller must NOT RUSH YOU, take all the time you want, avoid any unrelated chat with the seller, he/she might be hiding something. Concentrate on the car, forget about the seller untill you are 100% satisfied.
Its pretty common to find modified Mustangs, ask the seller for receipts and HARD proof of what has been changed, never take "his/her word".
You are the one with the MONEY, you have things under control, remember this.
Need advice on buying a Mustang. 
