Its a tough one and can only be diagnose correctly by carrying out some tests and then dismantling to determine the damage
Got to be worth at least running tests to see if it is the head gasket or water pump at fault - you may be surprised by the cheap cost if it is waterpump
However, loss of oil may be because it hasn't been topped up in a while and may just need an oil change (looking at it optimistically)
Let us know what you find out