"It's a house I'm buying (haven't closed yet)"[/I]
In that case there is more to it than "Dewinterizing", because there may be issues with equipment that are concealed by the fact that things aren't available to test. And you will be responsible for the cost to fix them afte you close.
I would test every element of the system. Every faucet, shower/tub, toilet, and drain. Run enough water to cycle the pump through several pressure cycles and be sure the tank is not waterlogged or damaged. Check the pressure gauge and pressure switch. Pump continuously to discharge at least 500 gallons of water to check the well.
Put at least 200 gallons to the septic tank. Around here (In Massachusetts) you can't sell a house with a septic system until it has been checked and certified as working. A new septic system will cost $10k to $20k.