At that price you'll do well...especially with a nickel gun. A Mark III is not an early Trooper by any means as the lockwork is totally different. A Mark III is an excellent revolver and will stand up to an awful lot of full-power .357 shooting.
It can be difficult to say what to look for as a well maintained and thoroughly cleaned revolver can appear new and unfired. Basically look for evidence of powder rings around the cylinder chamber face and a clean barrel and around the forcing cone. Depending on when it was made, Colt would have proof fired it with three rounds...one in every other chamber. If you see evidence of powder rings on the cylinder face with every other chamber that would be a good sign.