What regulator do I need for 500 gallon tank?

Just moved into a new house, the previous owner had a 500-gallon propane tank buried in the backyard. A gas contractor stopped by and said the tank is in full working order, however he doesn’t do installation. The tank at this point is not connected to anything. It was originally connected to a pool water heater, but the original owner said he replaced the heater to electric. The pictures attached I found online, but it is the same make and model, the only difference is the serial number.

I will be running an underground polyethylene line for about 80-100 feet. What size regulator do I need?

I’m planning on using a first stage regulator (like this one), then run the line and use a second stage regulator and finally connect my appliance which already has a built in regulator.