I have 2 late 80's 175 2.4L chrome bore mercs or a 1993 200hp 2.5L Mariner Offshore. 175's are the exact same weight as 200's. They have slight differences in the carbs, exhaust, intake. PM me if you're interested.
Do you need a 20" or 25" shaft? I have one of each for the 175's. Also these are light motors 358lbs . . . accordiing to NADA. I believe both are 87's but I'll check the serial numbers again.
Nat's 225 is also a good engine . . . they can be built up to put out over 300hp.
http://members.cox.net/aprilhorvath/...Project300.jpg
CHeck
www.screamandfly.com . . . there are plenty of guys that can hook you up with freshwater motors. BTW - My 20" engine is freshwater
Also if your 150 is a v6 . . . that's a good motor!! Those have the same crank as the early 260 merc racing motor and 5250 rods. It's pretty hard to kill those engines unless you seriously neglect it (which almost everyone does).