P38 Water levels cylinder head

Hi everyone new member, but owned 2 x 300tdi for about 4 yrs (its always good to have a spare) and now wondering about buying p38 4.6 hse 99 running on lpg, wondering if i have every 3rd week in the month to whip cylinder head off, or rip the block out on a saturday morning, Ive been to see a few now and im wondering where the water level should be in the expansion tank as the last one i looked at had sensor type rod going into the tank on the fill cap and the tank was dry but no issue on the dash and ran fine the water level was visible to the rear of the tank . And also how long it takes to change the head and can you tell which one has gone or do you just do both for fun. and any other advice on issues overheating.