Currently, there are the following types of outboard motors:
Conventional two-stroke Otto engines (with carburetor)
Two-stroke Otto engines with direct injection
Four-stroke Otto engines Diesel engines
Until a few years ago, the only engines available on the market were two-stroke engines with carburetor intake. This type of engine uses for its lubrication oil mixed with gasoline fuel (about 2%). Of the types of outboard motors described, they have the highest fuel consumption and are also the most polluting. They are currently banned from sale in Europe because they do not meet environmental requirements.
Otto two-stroke injection engines and four-stroke injection engines comply with environmental requirements and are the most commercialized types at present. They have slightly favorable fuel consumption for four-stroke engines, but two-stroke engines have the advantage of a somewhat lower price and weight.
Diesel outboard motors run on diesel, have lower fuel consumption for the same power but are very heavy, and market presence is very low due to their high acquisition cost.
Some best brands that you can find are Suzuki, Honda Outboards, Mercury, Yamaha Outboards, etc. Don't waste times looking for used engines that can bring you big problems, better invest in something safe and get a new one.