If it has an oil compartment, it has to be a two stroke. There is a rough and ready way to check. A four stroke engine has a compression stroke on every four strokes, whereas a two stroke has compression stroke on EVERY upward stroke of the piston. Remove the spark plugs, and turn the engine until you see the piston. Put your finger over the plug hole and turn the engine one revolution. Do you get compression? Keep turning, do you get suction and then compression again. If so it is a two stroke.
you can also follow the pipes from the oil tank and see where they go to? Yours should have a pipe from the pump to the carburetor inlet manifold.
On a new boat though, my advice for you is to take the boat and engine to a marine engineer for a service. The cost is minimal and you will get everything set up for the season, especially the carbs. You can ask lots of questions. The cost is about $150 and is well worth the piece of mind that you are not going to break down.
The critical bit you have missed from your post is the HP. If you Google
Mercury HP Serials
and you will end up at a site with engine details. Then google the same, but manual instead of serials. If you know the year of the boat, add that before 'Mercury'.
Use Evinrude XD50 two stroke oil. It is expensive but the best in clean running, non fouling and lack of acrid fumes. One $48 jug will last a season and a bit if you go out twice a week as I do. My boat goes to the marina on Wednesday :>)