I guess we'll start with the first question. Personally, I'd go with the T10. It's actually perfect height with the Reptile Basics RHP (if you need photos, let me know. I have a T10 set up with the RHP).
In regards to the RHP, I have a 40 watt but I have the T10 split so I'd suggest going with the 80 watt. The price difference is really not that big anyways, so you might as well splurge a little. When you install the RHP, I'd put it to either one side or the other. That way it creates a little bit of a temperature gradience. Ultimately, it's really up to you though.
If I were you I wouldn't even worry about the UTH. As long as you provide ambient temps around 80-82 F you should be just fine without the UTH. Most breeders just heat the whole room to about the same temps and call it a day. However, if you decide to do a UTH, I'd go to any hardware store and get furniture coasters and that should be more than enough for airflow.
Finally, I got both of my Herpstat 4s used (I have 3 AP enclosures). I got lucky with someone trying to sell one on here and the other I put a wanted ad on and what I was willing to pay on Fauna. I got a reply back pretty quickly. I had a great experience both times and they work perfectly to this day.