I just bought a 1978 Cobia 17 footer with a mercruiser 120.
Probably pretty close to the one you are looking at. Mine is rated at 140hp and runs great, I would say 40-45mph, has no problem scaring the crap out of tubers and skiers.
I did have to have the carb rebuilt this year, paid 19.00 for the kit and 50 bucks for the local hot rod shop to do it.
Have the guy start it, you can run it out of the water, but only for a very short time unless you have the wierd looking suction cup thing to hook a hose up to.
Try to get it into gear, then nuetral, then reverse. It should go into nuetral fairly easily, if not it might need a shift cable. Mine was streched out, no more adjustment, so it was a pain to get into nuetral at the dock. IE, made me look like a fool trying to dock and load the boat.
They aren't too hard to get parts for, but prices vary way more than I thought possible, so it pays to shop around.
Great fun, I love mine, but like many things, it is a starter boat.