Hi,
I did the same with my 5' reef tank last summer.
You would need to take as much of the water as possible.
Firstly make sure you have a RO/DI unit set up and produce as much RO water as possible before the move.
If not buying salt from the seller make sure you get some of the same brand and mix up at least 80 to 150 litres the more the better as you will get ammonia spike due to disturbing live rock and will need to do some water changes. Will assume for now that you know how to mix salt water to correct SG, and have marine test kit.
I bought a 2nd hand 3' holding tank so that I had a bit more time to set up the display tank.
Is the tank local to you or involve a long journey, is it reef, fish only or fish and live rock?
What livestock is in the tank?
Things you will need besides above:
Enough water containers (need to be water tight use bins bags and parcel tape to seal if not)to hold the water probably no bigger than 80ltrs each for manageability get a wheeled trolley to make life easier and van with tailgate.
Plastic bags for fish
Ammonia Lock
Several understanding friends water and tank weigh a tonne
This is how i moved mine:
Took enough water and LR to fill 3' holding tank would use 4' if possible, after transferring LR and water to holding tank (use spare heater and power head to circulate) went back for fish which seller had bagged while i was away, put fish in holding tank and went back for remaining water and empty tank.
Put display tank in place and filled with water(use a pump) and put in LR (but not all] from holding tank, try to aquascape as much as possible you can use sand from tank if you want, I decide against and added fresh washed sand later on or you can put new sand in at this point about 2'' Best to place LR on bottom or egg crate to avoid it falling, would add sand after water, however if using very fine sand your water will go cloudy.
Put in heater, powerheads,and other equipment ie skimmer, test water for ammonia and nitrites if these are high use fresh saltwater mixed 24hrs previously and left with powerhead in to aerate) to lower. Also make sure ammonia and nitrite not building up in HT. If ammonia detected add Ammo Lock.
When tank is ready transfer fish over.
If you cant use a holding tank fish will have to stay in bags or buckets until tank is ready.
Make sure you test water daily and change as needed.
Do not feed fish 24hrs before move also probably best to leave off feeding for a day or two after and feed lightly afterwards.
Think thats everything, let me know if need further help