The sport plane/kit plane idea is fun and all, but if it were my money, especially in an amphib, I'd stick with something more proven. These craft take an incredible beating! I rebuilt a bunch of floats and float planes as well as amphibs over 20 years ago (name a mfg and I've probably pounded a rivet or two in it) when I operated a rebuild/sheetmetal shop, and I've seen some pretty crazy damages.
Although the expense is higher, a good 180, 185, or 206 on amphibs or a Lake Buccaneer/Renegade are a much better bet for both longetivity and safety. As an IA and DER, I've been asked by both the FAA and some owners to get involved in the sport plane processes, and have politely refused. It's your choice and your money, but I've seen damn good Lake 200's sell in the recent past for less than 100K, and that's a helluva an aircraft for that kind of money. They're fairly fast, well-built, and give you land or water capability- something that might help replace that need for water with the boat you're going to sell.