Any second hand engine or gearbox is going to be a gamble.
I would check what they mean by reconditioned gearbox, because that seems pretty cheap. However no one wants the 20 box so that could be why.
Of course you can reuse your current clutch. You just have to be happy to pull the box off to replace it later on. Most people replace the clutch while the box is off, depending on the condition of it.
Before you get carried away I would want to be 100 percent sure it's the head gasket and not something silly.
You can buy kits to check if the head gasket is blown. They work by detecting hydrocarbons in the coolant.
You wouldn't want to pull it all out of lying to find out the previous owner put some weird shit I the radiator to fix a leaky water pump......