Bike or engine first? that is the question...
Id say bike first, then you can measure the frame to see what kind of engine you want, i've been building alot of 4 stroke beach type cruisers lately, they make for very dependable strong bikes.
I agree with everyone's advice on going over the entire bike before riding it.
those nuts you were reffering to are called
Nylock nuts, because they have a nylon insert that wont vibrate loose, and are worth the extra cost and effort.
BIG A$$ BRAKES! front and rear, as good as you can afford.
Have the wheels re trued, you would be surprised how loose they are from the factory.
Mind your final gearing(rear sprocket) you dont want to over rev a tiny little 2 stroke to much it kills engine life.
Ask lots of questions.
Good luck
BBB