Just 2 boats now, but they are my 5th and 6th ww kayaks. I've been through a few. With the exception of my Jackson Karma, I've bought them all used and broken even on them all overall when selling.
Class I water is really flat water, but moving flat water. You could easily paddle it in a walmart kayak.
Class II is where it starts to get fun, but is still pretty simple and low risk. I've not paddled beyond III+, have not been able to justify the higher risks and time it would take to be a class IV boater with young kids. The harder end of class III is plenty of fun in a kayak for me. I was paddling class III in my first season, when I had considerably more free time as I started boating before my daughter was born.
In short, if you're going to get into whitewater, do it before kids.
This was the hardest thing I've done so far, III+ to IV at higher water.
https://www.youtube.com/watch?v=oY_-yLp9ois