T3 is not bad at all. I just figured you'd go with a T8 because it's a little bit bigger and cheaper. The T8 is also a quick ship item so you will get it much faster than the T3 which might take 5-8 weeks. You can contact Ali at AP and she should be able to tell you the production time for both. Personally, I like the T10 thr most because it gives you a little bit of needed height for cleaning.
The 80w RHP is perfect. The 40 wouldn't have been enough. Since I have two of my AP enclosures split (I have 3 in total), I use a 40w in each half. In regards to the substrate, I guess coco husk might hold a tad more moisture just because they're actual pieces. You're right though. Once you set up the enclosure, you can decide if you need more humidity.