I'm not sure that a cool steam generator/mister type would fit, and I would be very pleased to be corrected. When I made an evaporating oil type for mine I found that it needed an extra concentric tube outside the brass "chimney" to reduce condensation and redirect any back into the tank. A fan might have helped, as well. Mind, when I trialled it, there was surface ice in places on Fairhaven, so it was **o*y cold inside that funnel.
Fitting a BEC should be no real problem, probably easier than fitting an RX battery, they are one of the simplest and cheapest circuits to make, and one of ACTions costs very little more than the parts to buy. Possibly less if he chucks in a case and the leads and/or terminals.
When sorting the ballast, it needs the superstructure in place. There is a lot of plastic there, and it needs due allowance making.
My first one was done with a two-part superstructure, the forward half including the associated deck. I had to re-arrange the funnel stays to keep life simple and have an easy de-mounting arrangement for the "aerial" rigging. A member of my club is doing one - has gone the one piece way. The two parts do need a substantial bridging piece to keep them fairly rigid when they being handled, but another time I think I would go that way.
I found a home for my electronic bits by using some of the spare sprues to form edge supports around the after deck hole. These support a false floor made from two styrene sheet panels. These are not fixed, they just sit there and support the RX and ESC, with plenty of room for a BEC and a switcher should they be wanted. When it was running an AM set, there was room for the scrunched up aerial, now its on 2.4, there is just plenty of room.