Marine: Boating, Sailing, PWC, GPS and Navigation - Fix the Merc or replace it with used Nissan?

I have been searching for a lower case for this '77 merc 4 cylinder without much luck. I think that I may have finally found one but here is the dilema. The guy with the part is a Marine Mechanic who will sell me the lower unit for $100 but of course it probably should have seals put in it and a pump. By the time that is done I'll probably have $300 in it. This same guy has an '83 Nissan twin 75hp with tilt and trim including the controls and gone over, tuned, lower unit oil changed, plugs, for $1000 and another 40hp the same way for $800 plus guaranteed.

My motor should be good if I get the lower unit on it, but do I do that plus rebuild the carbs and fuel pumps instead? The 75 hp will be newer and lighter plus have tilt/trim, but of course that is $1000.

Being a nube, I know nothing about Nissans, this guy is telling me that the 50hp Mercs were lowsy and heavy anyway, he also thinks that Yamaha, Nissan, and Honda are the best. He said if my motor has a distributor, don't put ANY money into it. If it does not, it would be OK to fix it. I have to look but I think that mine is without.

Your opinion?

I have a Nissan 90 TLDI,new in 2002. I also have run Nissan outboards for the past 25 years. I've owned everything from the 18 HP, 50 HP, up to the 90 I currently have. I have never had a bit of trouble with any of the motors with the exception of the 90, which had a recall on the injectors, but they were replaced under warranty. All I can say is they are a good dependable outboard.