Yes, AP will drill extra holes if you want them to. I needed an extra hole to run the power cord through for my Radiant heat panel. I just drilled a hole for that myself. PVC is extremely easy to drill through. If you don't mind drilling a hole, you'll be fine to do that yourself, then you can put it where you want it, if needed.

AP will send you a tube of sealant. They recommend you using it around the bottom joints and up the corners. This is just so urine, water, cleaner will stay inside the enclosure and it easier to clean. You don't need to seal the open surfaces of PVC, it won't rot. It's just to make it easier to clean and so mold can't grow.

Herpstat 1 - this depends on your heating setup. If you only have one heat source, this would be fine. If you're going with a T8 for one snake and are only going to use one 80w (or larger) RHP, you would be fine. Personally, I don't think heat tape alone is enough for a T8. I put a small snake into my T8, so I got the divider and am only using half of the T8 with a 40w RHP and heat tape. I have a Herpstat II, so I have a thermostat for each heat source.
You could get by using half of the T8 with only a 40w RHP and a Herpstat 1.