Ive just upgraded my 3ft to a 4ft and the 4ft was second hand and used as marine before it came to me - To clean it out I just used a wet sponge to wipe down the glass a few times and used a bit of a gross method to test it but basically I would run my finger down and lick it, if it tasted salty time to keep scrubbing if I was happy with it I stopped I also filled the tank up and once and drained it before doing the final fill and it seems to be okay so far about a month after the swap now.
But make sure you change all the substrate and media in the filters - also if its an external change the pipes and inlet/outlets just to make sure. A good scrub could clean the pipes but I chose not to risk it.
With the filter I just made sure it was as clean as possible scrubbed everything with a toothbrush and got all the impellers etc apart and made sure there were no salt/marine sand build ups.
The only thing I notice where salt is still present is when I get a bit of evaporation and water drips down the front of the tank between the hood and the glass it sometimes leaves a salt trail but that faded out over the first week so not a problem for me really.
Wills