Re: What size motor is best
In my opinion the factors you should look at are:
How much can you afford? Buy the newest, most pristine engine you can afford. If you're a decent mechanic and/or like keeping something running, less of a factor. If you're not a mechanic and/or don't like messing with it, buy as new and "ready to go", carefree as you can afford.
Is weight a factor? If you're going to be regularly taking the engine on and off the boat, how much weight to you want to mess with? If you're leaving it on there, this is much less a factor.
What is the stated maximum engine on the boat? With the above considerations in mind, go for the maximum. You don't need to use the power, but it will be there if you need it. Often two engines with different horsepower will weigh the same or almost the same, keep this in mind.
2 or 4 stroke? Personal preference - but the weight and cost factors may come into play, plus the fact that some waterways are or will be restricting 2 stroke engines. Not a factor where I live.