1, Yes, seems like a good deal as long as the tank looks like it is in good condition
2, How far is the relocation, hour drive, or hours?
3, convert back to freshwater, why o why would you want to do that
4, I'm not sure exactly what you're asking
There are lots of posts on here on transport, but basically you'll want to keep a lot of water. Assuming there is live rock you'll want to keep them in a tub submersed in the water, preferable with a heater and air circulation. You'll bag the fish and have them floating in the heated water. It is recommended that you remove all the sand and start out new. As long as you have the live rock, you can use dry sand, just be warned that it may cloud up a lot in the process. So you'll add the sand, water, rock, and fish. As far as filtration, your LR
is your main filtration. Power Heads to get the flow you need in the tank.