My SL spares car has turned up.

I bought a 500 SL spares car. It was described as overheating after running for about 5 minutes.
I just started it for the first time tonight and sure enough, it boiled within a few minutes. That was fine because I only wanted to move from the driveway into the garage. I turned it off and had a bit of a nosey around the Engine. I was expecting to find Mayo in the radiator cap, Dipstick and Oil cap but they where all clean. I also expected to find a bit of water still in it. Nope. It was more or less empty.
Curiosity got the better of me so I filled it with hot tap water right to the top and started it up and waited................and waited.................and waited. I left it idling for about 20 mins and couldn't get it to boil. I left it to cool and checked the water level. No change.
Surely it can't have been as simple as low coolant. I will flush it right through and sniff test it just to get to the bottom of it. If it all turns out good then I guess I will just use the complete Engine and Box in My good car instead of stripping the front Timing cover off.