Might be hard to clean as a Mash tun but you cold make an insulated lid and use a bag that hangs from the top to put your grains in and drain using the spigot. If it's jacketed you can't heat it. But you could also make a lid and use it for a fermenter.
I use an air compressor to push pressure past the valve. Add some star-san pressurize it 10-30 psi. Turn it upside down see if anything is leaking if not leave it for a day and come back pull relief valve or push down on one of the ball valve to see if air comes out. If it's still under pressure...