Outboard motor help

So i went to look at a 22' skeeter bayboat yesterday. It has an '08 mercury (yes i know, I like yammys too) 225 optimax. Hour meter shows 106hours. On inspection of the motor it looked pretty clean. No rust or water spots on the bottom engine area itself. There was rust on the air compressor and some rust on a screw underneath the air compressor which made me think there was a water leak some where. There was also rust just on the bolts on the port side of the housing which is directly under the air compressor. The owner "says" the spark plugs only have 8 hours on them. We pulled the plugs for the compression test and 3 of the 6 were rusted pretty good. I was busy chasing my son around and didnt see where he pulled the rusted ones from and if it was from same side. He said they were not from the same side tho. We were unable to do a compression test bc he forgot to charge the batteries and the engine wouldnt turn over. I hired a mechanic to go look at it but im 2nd guessing myself on this buy. To me its either a leak around the compressor or a blown head gasket but im no mechanic and thats why i come here. Thoughts? Here is a few pics of the compressor