Hi bb6,
First up, congrats on you new camper! You can't go wrong with a Trackabout!
Best advice I can give on this is to have a chat to JT himself. The man's got more experience with these things than most mere mortals, and knows what works and what doesn't. Plus he's not afraid to talk you out of expensive options if he thinks that they won't suit you.
When we bought ours, my mindset was to get the stuff done that would be a pain/impossible to re-do later on, but leave pretty much everything else. So if you're getting the trailer powdercoated, then certainly have a think about any holes or mounting spots that you might like drilled/welded, because it's very hard to do that later and not have it look like it's been added on. If you can do it beforehand, then it all just gets powdercoater together and looks ace.
Given that you've got 4 kids, I'd probably stick with the 12" tent. They are more work to set up, and I do like the 10" tent we've got, but I think you'll really benefit from the extra space. Especially with the enclosed annex - it adds a lot more usable room.
And seeing that you can do the electrics yourself, I'd definately be following Humbolts suggestion and asking JT to get all the electric mounting/access stuff ready for you before finishing. He was able to do it for us, and it's going to make a big difference to the finished job once I can get around to doing the 12v fitout. Also as Gaz has said, get the 2 battery trays even if you only want to use one initially, having the second one there will make it wasy to upgrade later.
Water tanks - I'd definately spring for the provision for the second tank, even if you don't actually go for the second tank straight away.Same as above - putting the extra mounting rails under now and powdercoating them is easy and straightforward compared to retro-fitting them later.
If you're looking at doing the big lap, easy storage becomes crucial to having a good time - a place for everythign and everythign in it's place. It might be worthwhile have a chat to JT about the Drifta storage draws in the tub of the trailer, or other similar solutions to make internal organisation easy.
About the only other thing I'd try to talk you into is upgrading to one of the Extenda trailers
, but I probably don't have much hope of that. It sounds like you're on the right track though! Enjoy your new camper when it arrives! Lucky ducks!
Congrats!
Matto