Peugeot 103's are great mopeds. I currently have a 1982 103 model, and had a 1978 103. The engine and drivetrain are very simple, so that makes it easy to work on, and less stuff to break.
The major thing to look for on any Peugeot moped is rust in the gas tank. Take off the cap and look in there to see how much (if any) rust is on the sides of the tank. If there's a lot, avoid that ped. My old '78 frame had to be junked because the gas tank was rusted so thin in parts, it couldnt be fixed properly. My '82 has no visible rust in the tank. I got very lucky :D
On a 103 model, look at the rear sprocket. There is a number etched in the sprocket telling how many teeth it is. If it's a 59 tooth, you're probably going to see about 25mph on flat ground. If you see a 48, then assume about 30mph.
If you're looking at a 102 model, check the frame where it meets the gas tank at the bottom. They are prone to cracking and leaking gas in this area.
The only other thing I would tell you to check is the wiring. Look and see if anyone has been rigging stuff up, cutting wires, splicing stuff, missing bulbs and switches, etc. The electrical system can be a major pain if you're new to working with electronics.
hope this helps,