I'm curious, in the ad, did he advertize the engine as 1,500SMOH or 1,500TT?
Because if it is 1,500SMOH and 5,100 TT that seems more legit me. If the engine isn't factory overhauled or new, it won't be zero-timed. If it is the case where it is 1,500SMOH/5,100 TT I wouldn't feel too weirded out especially if you had an A&P do a pre-buy inspection on the thing. Typically in ads you only see the TSOH times and not the total time on the engine unless it's really low.
The O-300 is smooth as silk but a little under-powered I think for a 172. You have those two extra jugs to deal with.