Our frost line is 48" (under grass maybe 18-24") I would need to go with a low profile pressure tank (some suggest small tanks anyway) . Does anyone think it would be possible to get by at 36-40" deep with tank on bottom and control box center and maybe a foot or so of foam on the underside of cover.
I think this is your best idea if you can seal it to keep the water out of the pit. You could install a Pside-Kick kit and would only need 14" X 14" X 24" to fit, which includes the tank. Use a 2 wire pump and you won't need a control box.
Temperature from the earth will help warm the vault a little, an if there is occasionally water used, it will also help keep it warmer. A tiny little heater with a thermostat works great as long as the power is on. But when the power goes off the heater is no longer of any use, which is why it works better in a vault than an insulated well house.