Problem with LEDs is you can be buying a real lemon.
If you can help it, go for an LCD. LCDs take more common parts and upgrades. Plus at the time when LEDs were being made WDP's QC wasnt up to scratch in my opinion.
Many LEDs have serious problems, the LED I bought was completely twisted (it looked fine) and this caused it to eat paint. WDP told me it would cost more to re-mill than to replcae, so thats what i did.
On the other hand you find the odd LED which is a peach, and shoots like a dream witout problems.
What you should check:
Look down the breach and check it all lines up.
Make sure the guy whos sellin it ya can put a few pots of paint through it without breaks.
Open the breach and look for excessive wear on the hammer, the bit the bolt pin sticks into. When you close it the breach should snap closed nicely.
Err theres not much else that can go wrong that cant be replaced.