Call the propane suppliers in your area, and find out what they charge to install a tank and connect it to your house. You really don't want to try to reuse anything, and they won't want to either.
By "hot water heater" I assume you mean a "boiler". (a boiler is a big water heater that is used to heat an entire house; never mind that it doesn't actually bring the water to a boil).
Whoever is installing the boiler should also install "black" (steel) pipe from the heater to somewhere outside the wall. Depending on the size of the boiler, it will be between 1/2" and 1" i.d. pipe. The propane company will typically install a secondary regulator on that pipe, which will then be connected with 3/8" or larger tubing back to the primary regulator on the tank.
After the tank is installed and connected to the boiler, the boiler installer will need to come out and adjust the boiler.
Check with your neighbors who run propane to get recommendations on suppliers. Propane suppliers vary considerably in price, reliability and integrity. Most will install a tank for a nominal fee and waive the rent if you meet a minimum annual purchase. If you're in an area with no competition, they may insist that you buy your tank or pay rent.
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