Insulated water troughs is brilliant! I don't know if I have the skills to add solar power (I will have to study that great how-to very carefully) but I can see how putting in a water heater without insulating the trough would be like throwing money out the window. My horsey neighbors, in fact, just commented to me that heating the water in the pasture was a big expense.
And I do have a trailer with dump function that I can hook up to my ATV. I thought I'd still want a small wheelbarrow or cart that can easily be pulled, even by a child, for daily mucking out. I'll use my ATV to haul the manure away weekly to the compost piles. So do you think that I still need the 1200 lb one? There isn't much difference in cost so going from 600 to 1200 lb is no big deal. I'll have to see how big they are though. Keep in mind we're only planning on getting 2 horses.
If you're only getting 2 horses, then just haul out to the compost pile daily. You'll learn quick enough that doing a chore twice is just not on the agenda (daily to a small pile, weekly moving the small pile to the larger compost pile). I have 8-10 horses most of the time. If they are in for a couple of days and make a real mess in the barn, it will take me 3-4 trips out to the manure pile with my fairly large trailer. Just overnight I can do in 1 trip, so for 2 horses I would park the atv and trailer in the barn and have the kids just toss the pickings in that trailer, and you can haul it out to the heap daily or every other, depending on how many flies you get. Depending on the kids' ages, driving the atv can be a good learning experience for them.
I would still get a wheel barrow. The 1200 lb one is only 2 ft X 2 ft X 3.5 ft (25 X 25 X 40 in.) and the 600 is only a little smaller at 20 X 20 x 40 in. and I find that a lot of items that you would haul will be roughly 2 ft wide, so easier to maneuver in the 1200 lb one.
The "solar" heating is not real tricky. I use the black rubbermaid stock tanks, rather than the metal ones, and I put a clear plastic "window" on one side of the trough. Just make sure the sun can hit it but horses can't. Instead of floating heaters, I use the plug heaters that stay on the bottom of the tanks, I just plug them into a strip that I can turn on and off as needed. Down here in OK, unless the windchill gets really bad, I rarely have to turn them on.