I looked at the ad, didn't notice anything about it having an 18hp engine, he mentioned an 18hp Magnum he had that he will sell though. I'd never pay the $500 for the other engine unless he had proof it was a recent rebuild.
I think I paid around $150(maybe a bit more) for aftermarket pistons and gaskets for a M18 this winter. Another $50 to grind the valves and seats. Another $40 to replace the fuel pump. My bores were in spec so I only had to hone. Boring was going to cost me another $50. Didn't have to split the case, so I'm hoping the crank and rods are in good order. Just tossing out some ideas so you will get the feel for what the 2072 may cost you. Of course if you open it up and the crank needs turning, rods are shot, well you get the idea. My situation was I already had the mower I was working on and had already dumped a bunch of money into it. You'd be starting out $900 in the hole to begin with.
I'm thinking you could easily have more money in this by the time it is fixed than what it would sell for running and mowing grass. If you do go to look at it, check over other items like driveshaft--front to back (you can drop $200 on rebuilding one of them). Check the deck and mule drive over really well, easy to lose another $1-300 rebuilding those.
Not trying to talk you out of this one, just giving a heads up of things I have personally ran in to. My 1811 started out as a $3 or 400 CL buy, but it didn't leave the shop that way!
