I've bought a couple for my old 88 4.3, paid in the range of 160-170 at a local marine shop that sells aftermarket stuff (Sierra, MES). Installed it in about 1/2 hr. Just make sure you disconnect the batt neg cable first before disconnecting the pos cable at the solenoid. The bolts are way under the rear of the engine, 9/16ths if I recall. I used a big 1/2 drive ratchet wrench to get em off. A tip to make it easier, is tie a line round the starter, and have a helper hold it up for you, so you don't have to hold the starter up with one hand and get the bolts out. They are heavy when you are reaching that far back....
IF there is a front mount bracket, make sure you use it, that helps steady it so the mounting bolts don't get stressed and break (that would ruin your day).
As far as part #s, look at the on line Sierra catalog and you can get the # there based on the engine you have. Or, look up your engine at
http://www.dougrussell.com, get the original part #, then cross reference that on the Sierra site...Arco and MES also sell new Marine starters....