Congratulations on your first boat. For $400, I don't see how you go wrong if you are somewhat handy and don't mind getting your hands dirty. If it floats, doesn't leak and has most of the necessary equipment, I don't see how you go wrong.
When choosing an outboard, you'll need to consider the weight of the boat, characteristics of the motor mount (is it rated for 4-stroke, what is the vertical rise), 2 vs. 4 stroke motor, new vs. used, length of shaft on the motor, typical size of waves, and tidal flow when making your choice.
I had an O'Day 222 and used a 6 hp long-shaft 4-stroke Tohatsu (2005). It pushed the boat at hull speed on smooth water, and near hull speed even with some small waves on Lake Michigan. Even in big waves it moved the boat well. I replaced the motor mount when I got the motor. I had a little cavitation issue when the waves got big (ie., the prop comes out of the water as the boat crests a big wave), but by and large it worked fine. Just be careful with the ethanol mixed gas. It can mess up motors if you let it sit too long. I tried to use pure gas when at all possible.
Good luck and welcome to the club.
Dave
O'Day 25