When you take the sand out to clean it if the sand smells bad dont use it. obvously you will need to keep the sand wet with saltwater. If the sand is dry then I would take it out and run it over a screen to get any clumps out of it before using it again.
TO clean the tank use a bleach/water mixture or use hydrogen peroxide/water mixture. If you use the bleach/water mixture make sure you rinse the tank good after cleaning and also let the tank airdry.
I'm also looking into a simple sump primarily to hide away the heater and skimmer. Can the 25 be used as one without modifying it in any way?
Sure. You would proably be best to split the 25 up into a few compartments with a glass sheet syliconed in the tank but if you want to do it with out that you can. The return pump would sit on one side of the tank and the supply lines from the overflow would come into the supm from the other side.
The skimmer and rock will be ample for filtration along with your sand bed. You might want to bump your rock amount up to say 75-90lbs.