Por-15 is basically a paint, available in couple of different formulations. It's a system in the sense there are products used to prep the surface but you don't have to use their products (degreaser and an an etching solution). The basic prep materials aren't expensive. It's not a system as you might expect with a two-part epoxy "system".
As with any paint, a couple of coats is best. You can easily repair the holes with Por-15 paint and fiberglass matt, either their brand or any other. The silver paint that has a higher solids count is recommended for the fiberglass but if the holes are small I don't think it matters. I repaired a severely rusted floor pan in the cockpit of my E350 with Por-15, fiberglass and some pop-riveted sheet metal patches. I used the fiberglass to cover the smaller holes:
https://www.sportsmobileforum.com/fo...diy-27589.html
A lot depends on the severity of your rust and holes. Rustoleum might be all you need but for more significant issues, I'd stick with Por-15, it was recommended to me by a friend who professionally restored classic cars and hot rods.
If you could post some photos, we could give you some more specific advice.