How long ago did he break the skeg? If it was recent and the reason he's selling the boat the lower unit may be compromised. A broken skeg is also a safety issue, many recommend having it cut of and welded but that in itself is also a safety issue if you speak to Mercury. I would add in the cost of a new lower unit for this reason or as a good reason to talk down the price. Otherwise if everything else checks out good with the motor, it seems like a pretty nice deal.