1. Truck bed liner is expensive but that is totally up to you.
2. I got my windows from a local salvage yard but you can get them off Ebay....google is your friend.
3. Reinforcing seams is always a good idea. Depends on how much you are going to beat it up taking it off and on.
4.
5. Any local upholstery shop can do your cushions but you might be able to get a better deal on line. Again Google..or make your own.
6.
7.

I think that is up to you. I would check out reviews online before I spent that much money for a gallon of anything.
Oh hey and

to the playground.