Swapping From Marine Tank To Tropical.

You don't use live rock on a tropica tank, so you can dispose of that.

The tank and filter casing etc can be scrubbed to get rid of the salt. But I would throw away the filter media and get new. I would not be confident about removing all the salt from sponges, noodles etc.

Milton fluid is a recommended way of sterilising tanks. It is bleach with nothing likely to hang round in the tank to kill bacteria or fish. After use, wash, wash, wash and then soak in water with a lot of dechlorinator added. Bleach is just chlorine. If you can find a supermarket's own version of Milton, that would be cheaper 

Since you already have a mature tank, cycling the new one will be easy for you. Will you be running the Trigon alongside your 80 litre, or is the new tank replacing the old one? The way to set up the new one depends on what is happening to the 80 litre