Sounds like your getting worked up about it, Mark. I like to have fun with it. This is a toy afterall. Enjoy the process. Test drive an I/O, an outboard & an inboard. Bring at least one of your kids along & have them ride or ski while you drive. As long as you tell the seller your intentions & offer to reimburse them for gas, most sellers are fine with going out for a test ride.
While your having fun with it, pay attention to how the seller gets the boat on & off the trailer, starts the boat, drives, etc. He will have a routine he follows everyday. Everyone's routine will vary but includes the same details. You will learn a lot the first few times out.
If $9k is what you want to spend for that size of crew, most any 19' - 20' boat will be fine. Or it could be a downpayment on something nicer. If your kids are serious about their riding, even potentially serious, then an inboard might be best in the long run. If you think this first boat is something you might want to run for a couple years, then upgrade if you get serious, an I/O will probably fit the bill just fine. Outboards are not typically used for ski/wakeboard boats.
Relax & have some fun with it. Come back with some more specific questions & I'll give you my 2 cents.