I have always built my tanks by assembling everything at one time. It is really the only way to get the tank squared and the seams nice and even. I also pay very close attention to the amount of silicone between the glass panels, one mm is plenty. Building a tank upside down is a good idea for tanks where the side panels sit on the top of the glass. My tanks have always been built with a raised bottom panel, so the sides circumvent the bottom. Doing it that way eliminates the need for a cushion under the bottom since it is not resting on the top of the cabinet. In the OP's case it must be re-assembled in the same manner that it was originally constructed. If the glass is in good condition there is no reason not to use it and same a bundle of money. Only expense would be a couple of tubes of quality silicone, some very fine steel wool, razor blades and a can of acetone. One additional thought/trick. The best method I have found to separate the glass panels is to use a razor blade removed from a disposable razor. They are very thin, sharp and flexible. I have tried piano wire and fish line but for me this worked the best.Cubes are a different animal.. if you reseal it you have to do the whole thing at once. Preferably with it rimless you do it upside down.
One thing to add. Its salt water. You'll have to pour water over the spill and suck it up with a wet dry vac. Than dry it out. If you don't the salty will always attract moisture and cause you lots of problems. Yes, wash it down heavy where the leafy was with fresh water.