Hey Scotch,
The engine and trans together is able to come out in one piece. However, together they are very heavy and long. ( I would estimate my RD28T and trans would come in around the 600 - 700 Kg mark)
I took out an RD28T with the transmission and transfer case connected and took me about 5 hours by myself (so get some mates to help you)
Swapping the engine with another ZD30 would be the simplest method as you dont need any engineering certs to get it registered. Just a Blue Slip to change the engine numbers on the rego.
If you are going to do the engine swap yourself, I would advise taking your time and 1000's of pictures of all connections. I would also label each connection to the engine from the wiring harness to the fuel and vacuum lines.
If you can, get an engine and transmission together. I am having issues with my clutch and after installing the engine and getting it running I now have to drop the gearbox to fix the clutch.
Cheers,
Rob