Looks good!!!!!
Curious that you extended the bottom of the stand, but curious why you didn't "cap" or finish off the bottom section with a piece of 2x4 across the bottom in back....(if that makes sense). I'm assuming the sump will sit towards the back meaning the additional depth was needed for the sump to allow it to sit back an allow the skimmer cup to be removed......
I would now put my cross bracing in the top section where the tank will sit on and also in the bottom where the sump and equipment will go.....Then I'd cap it off as you had suggested with ply, and you could seal it with epoxy. I think Corey had mentioned about somewhere that carries epoxy paint.....Probably a cheaper alternative would be a good coat of latex paint.....I like to also caulk the joints(where the wood meets another surface). I would just put a couple coats of latex paint on the inside....I also like to use FRP board as well in my sump area.....With the FRP board again it can be sealed and contain water spills to a degree and can be wiped clean easily.....With epoxy and a good epoxy job it will hold and contain water and would be easy as well to clean up depending on the epoxy used.....Check Lowes or HomeDepot for the epoxy. Not sure, but a good start.....
Adding a light inside the sump would be a good idea....I've been toying with that idea as well, but haven't done anything yet.....Actually I wanted to use LEDs with activated switches so when I'd open the stand doors the lights would come on....Again not really needed but neat....Fan could be useful, but not sure absolutely necessary.....Shouldn't be an issue with humidity or such....Again totally up to what you want to accomplish.....Electrical would definitely be a big plus IMHO.......If you search around there are tons of neat electrical junction box setups......You might look around and see if you can find some American DJ control boxes....Already to go you just need to install them.....Have separate switches to independently control each item.....