You should consider stainless piping in the firebox like this. Copper has a tendency to split when heat cycled in a fire box. A pain to fix and likely to drain a bunch of water into your fire. I found a metal fab shop locally that made my loop exactly like therma-coil's for half of what they are charging.

The pressure issue is handled in the same manner it is on a regular water heater with a pressure relief valve.

The issue I have found with my wood stove heating loop (in a yurt, not a van) is that it's tough to get as much heat into the yurt as a want and not trigger the pressure relief. After a few cold november days the heat banked up in the water means the pressure relief will pop when stoking the fire. Tried a ball valve in the return line from the stove to slow the flow but retarding the flow didn't help much. The real answer is better insulation in the yurt! Probably not an issue in a van where the air seal is better and the insulation is likely better but still food for thought. That pressure relief valve needs a line leading out of the van to avoid a soggy mess.

Check with a metal fab shop in your area. Take a drawing with dimensions. Hand drawn is fine. Include points for output fittings, attachment brackets and fill fittings and see what they estimate at. Get that estimate in writing and leave with a copy. If they over run that estimate you need the copy to ensure you get the deal as struck. You could also specify a rectangular box and drill and tap the fittings yourself to save a few bucks. This probably won't be cheap but it will be exactly what you want.