Prop size will be WOT RPM dependent. What are you currently at with your 15p? With your 17p?
I would wet sand and buff instead of paint or wrap. If you've got chalky gelcoat, start wet sanding with ~800 and go lower if needed. I had to go way down (300, I think?) then work my way back up to 1,000 grit. After you have all the chalky gelcoat sanded, buff with a rotary high speed buffer and wool pad using Aquabuff 1000, then again with Aquabuff 2000. Next, hand wax it (I use Collinite wax). You'll be surprised how far you can bring it back to life. It's a lot of work though.
I would take the hydrofoil off. Mine broke off my drive (and took a chunk with it) this past weekend.
Trim gauge just gives you a general idea of where the drive is at. I have one. The dashes / numbers don't mean anything significant, but it's nice for me to know about where I'm at with my trim. I'm using the Sierra Eclipse gauges.