Nice work. That paint job is awesome, and you couldn't beat the price for the paint. Actually, you seem to keep getting great deals all around with your boat.
Please explain the gas price!
I simply laid the floor on some aluminum that I riveted to the floor supports. The aluminum creates an even/flat bottom support for the plywood floor. It is a large, roughly a 4' x 6' piece of 3/4" marine plywood that fits snug between two bench seats. My boat is a 14" semi v-hull Alumacraft...
I put down some marine plywood in the bottom of mine. You could easily put down carpeted plywood if that is your preference. There are many examples on this site for different ways you could do the bottom.
As long as there isn't a huge discrepancy in the boards, you will be fine. As the previous post states, screwing in the plywood will take care of them being a little off. Before you screw down the plywood/deck. Put it in place and walk around on it. If there is any sagging, you may want to...
That's the way to go... use a fine grit to sand down the paint down to where you no longer see the brush strokes (or just less visable). Then just paint over everything using mainly a roller. There is no need to strip it bare and start fresh.
Remember, you are seeing the brush strokes because...
I think that is a little high. I found a used trailer in the local bulletin board for $125. You may be able to find a good deal on a used one that may take little to no repair.