As a relatively newish SIBBER, I have a 6hp merc and it's fine for what I need. I think you'll find that whether you go for an 8 or a 10 the weight will be the same as it's the same engine block. I can manhandle my 6hp with little problem as it weighs about 27kg. However, the engine block for the 8 or 9.9 is around 38kg and that is a significant weight difference if you're carrying it any distance and it's an awkward shape to carry easily.
I know the 2 strokers will be along to convince you about the merits of those engines.