I'll try to answer your questions; but if you want to try a Wave for yourself, you're welcome to drive up to Indy and take mine for a spin anytime.
1. There are white after market sails available, but I think the factory standard is "Spirit" these days. (The one in back.)
2. Three or four people is no problem for floatation, but the more weight you add, the slower you'll go.
3. I think a new Wave is about $5K; used ones are less, but you don't see many used ones for sale in the midwest. People hang onto them.
4. I store mine mast up, and I go to a resort in Florida which has always stored theirs mast up, for 5+ years, with no problems.
5. I flipped mine for the first time last weekend. It was easy to right. The only tough part was climbing up on the slippery hull to stand up on it.
6. I'm not sure how it compares to a 16 downwind. Granted, it's not the fastest point of sail, but if you keep your weight forward & centered, it will move right along.
7. It's not going to tack as easily as a Sunfish or Laser, but I think it's easier than a bigger cat. Just turn slowly, keep your weight on the old windward side, sheet in hard going into the tack, then ease the sheet as you cross, and finally shift your weight. Jibing is much easier on a Wave than any monohull (or most cats), with no boom to contend with.
When I was shopping, I talked to Strictly Sail in Blue Ash, and they were pretty firm in their price. You can try Sailboats Inc. in Indy, or contact Skip Kaub (who organised the Wave Midwest Championship last weekend). He has assembled an armada of Waves, which he may sell now that the regatta is over.
Bill