First and foremost, Ali (the "salesperson" at Animal Plastics) is INCREDIBLE to work with. She will absolutely not try to steer you towards a sale in favor of giving you the information you need to make the best decision for you. AP has some of the best customer service in the business.

1) There is no slot for the divider - it can be placed basically anywhere you choose within the entire 4ft length of the enclosure.
2) The T8 is plenty tall - no reason to gain the extra few inches in height, IMO.
3) I prefer the sliding glass doors myself.
4) Heat tape is a good choice. The 11" wide strip that AP offers is well suited to a divided T8.
5) Humidity retention is variable based on the substrate you choose to use and the room you keep the enclosure in. I live in AZ where it's pretty dry and use corrugated wrap as a substrate, so I run a humidifier on a humidistat in the room to keep humidity up. Others have different results with different setups in different places.
6) Skip the basking shelf for BPs, IMO. It's entirely up to you, but it likely won't get used much.
7) Cage casters are wheels to put on the bottom of the enclosure so you can move it around if you want to.

Here's a relatively recent picture of the two T8s I have. Please excuse the fingerprints on the glass.