Total posts : 45366
“i was worried about 1) the power line acting as an antenna and picking up unwanted signals and 2) a long cable after the power transformer having resistance and resulting in a lower voltage to the transmitter.”
You can put a line choke on the power cable feeding the box upstairs and that will kill any RF interference on the line. Put the choke on the line at the point of entry inside the box.
Unless your power line is ridiculously long, I would not worry at all about any voltage loss with the 120vac feeding the box. Just make sure everything is weather proof and water tight.
Do frequent inspections of the box for any signs of leakage into the box or worn/broken feed through seals.