I'd recommend a 15,000 BTU unit and never look back. You can always cut it back if it's too cold. None of these aluminum trailers are insulated that well and aluminum has a great heat transfer coefficient (which in English means if it's hot on the outside, it will be hot on the inside).
I replaced the 13,500 BTU unit on my 34 footer with a 15,000, and painted the roof white, and it made all the difference in the world. I went with a Carrier AiRV, but they are no longer made. So I guess you are looking at Dometic or Coleman. Both are good.
As for the drip pan....it's not a big deal. You can let it drip down the side of the roof, along the rain gutter channel (well, Avion has one anyway
But if you must have the drip tube hookup, be sure to ask.
Best of luck,