I had an Evinrude 25HP twin which was great, good starter, good fuel comsumption, quiet, just that it was short shaft, & I needed a long shaft, otherwise it would still be on the boat!
Doubtless ChrisP will have detailed knowledge of these engines, & give you the low down on them, mine was fine but "one swallow doesn't make a summer" there might well be a history I dont know about.
This is a really old engine, 1960's, they are heavy and bulky for the power output compared to more modern engines.
Negatives first. The alloy they were made of was not very corrosion resistant so those used in sea water can suffer very badly from corrosion. This is internal as well as external and if the engine was not flushed through regularly with fresh water it could be terminal. They were orriginally fitted with brass thermostats and that was a recipe for corrosion in itself. The new ones are in stainless but check out the price, ££ Depending on when it was last run it will probably need a fresh impeller, these are still available I think (about £22 for the last one I got from memory) but the complete impeller service kit is not. They were held in by 4 bolts and they tend to shear very easily due to the corrosion mentioned before. (The one I did sheared all 4 bolts and I had to drill them out oversize and re bush the holes with bronze inserts which I re tapped to orriginal size) They ran on a 25:1 mix so your bill for oil will be heavy. Some parts are no longer made so if ay of these fail you will be in a world of grief. Most tend to be found at the back of someones shed after years of neglect and then sold for a couple of hundred. If they have been left for a while there is another common fault, they had coils under the flywheel, these swell with the heat and crack the epoxy they were potted in. While the engine is dry it will run OK but as soon as you get in the water and the coils get damp it will start playing up. New coil time £££
Positives. If you get one that has really been well looked after, been run regularly and serviced properly it will run well. If you are really handy with engines and have access to an engineering workshop to make bits yourself then it could be a good buy.
I would personally go for a more modern engine as the availability of bits for these old motors will only get worse and you end up paying through the nose to keep em going.
Gareth, reading between line 1 & line 25 of ChrisP's response, the answer is dont touch it with a bargepole! My Evinrude was a 1998 so a different bird altogether, the one you have your eye on is a science museum piece apparently! Hear the word of the "oracle" & leave it where it is!
Blue, I try to answer questions for information, directly. ie what is the oil -fuel mix for my engine. When someone asks for an opinion I try not to be biassed, so give what information I have based on personal experience, What is ideal for me may be the worst thing ever for someone else. In this case the engine is 40 years old but there are some people who have classic engines matched to classic boats, these are serviced meticulously and run perfectly, it wouldn't be fair to them to say the engine was rubbish as a generalisation. It could be one of these well serviced engines Gareth is looking at and he may have to facilities to maintain it.
Also if I was to liable product here it could lead to legal trouble for the site as I have an official connection with WSF as a mod so I try to qualify my opinions with examples.