Talk to your propane supplier. I haven't had propane for fifteen years, but as I recall we had two regulators, one on the tank and one at the house. Plastic line, about 3/8" ID, was buried between the tank and house, and iron pipe was used after the house regulator. I hope you're not planning to take the low pressure gas from inside your house and run it through a 1/2" line for 250 feet. Tell your propane company what you want to do and they should be able to set you up with a much cheaper solution than copper, and one that will work. They may even be able to run the line for you without trenching.
I do know that 1/2" pipe is going to be on the small side for a 150K Btu heater. 3/4 inch is typical for a furnace connection. This is for the final connection, after the second-stage regulator.
A 140' buried air line sounds like trouble to me. Unless you're going to bury it below the frost line, I'd drop that idea.