I'm glad to see you're doing your homework ahead of time. That will make a big difference. I would go with the largest enclosure you can. The extra height will allow for plenty of climbing room while still giving enough room for proper lighting. Out of the Exo Terra line I'd get the 24'' tall but you can probably build one larger more cheaply. 4'x2'x2' is usually the minimum size recommended for an adult dragon. I'd build it even a bit higher if possible. Modern T5 lighting can sit about 12'' above the basking area and still provide great UVB exposure. This is a popular model to build
https://beardies.dreamwidth.org/2145.html
You'll need to play around with different wattages and see which one provides the best temperature gradient. You can either try adjusting the height of the light or use a lamp dimmer or dimming thermostat to control the output. I use a 90w GE PAR38 halogen flood light and it works well at greater distances. Having accurate measurements on the basking surface and cool sides is important for adjusting the lighting properly.
You may also want to get a roach colony started ahead of time so it's ready to go when you get your dragon. Having a renewable source of bugs is really helpful when they hit a growth spurt and start eating a lot.