Firstly, welcome to the site!
I would scrap the lights and go with 11" heat tape or an undertank heater. Lights are very inefficient at heating, and are also an increased fire hazard. They are best left for basking lizards and chelonians. Just place it beneath the cage, set it up with a dimmer, and you're all set
You shouldn't require any additional vents, in many cases even just the space around the doors is enough.
As for finishing the cage, I'm a big fan of epoxy. I find it much easier to work with, you use much less of it, it doesn't offgas when cured, it is 100% safe when cured (can be used on food and medical surfaces), is tough as nails, and is totally waterpoof (unlike urethane). The kind I use works out to be less expensive than the cheapest polyurethane. Just locate your local place that specializes in protective surfaces, and they will best direct you to a product