It doesnt matter Harry.
It depends on the setup of your boat, as you are correct in that you need to balance out the weight, so consdier where your batteries etc are.
Another tip is to make sure you back the bolts with a piece of marine ply or similar, just to spread the load.
FYIW, mine was mounted high up on the portside, it worked fine here. Although I have seen many 165's with them mounted low in the engine well alongside the main engine.
As for 2/4 strokes, if you only want it as as back-up go for a 2 stroke, simple to maintain and cheaper. If you are going to use it (trolling etc) then a 4 stroke is the way to go. 2nd hand 2 strokes are also easier to find.