I like to over engineer things, so, I'd use 2 x 6's for the top, & 2 x 4's on the bottom for th ebase, this way you won't ever worry about the "sag". 2 x 4's will be plenty for the "legs" also. You would be surprised how much weight they hold on end. I built a 2 x 4 stand for my 75 Gal tank which has the same footprint as a 90 gallon tank. But, I used a center leg in front & back... To be honest with you, the 2 x 4's should be OK for the entire stand, seeing the ones on the market come with either particle board or 1 x 6 pine (or Oak). Most of the ones I have seen are glued & stapled together. NOT something I would trust with almost 1000# of fish, water, & plants on... LOL!
Good luck & keep us posted!
As for the sump Q's, I can't help you, I use canisters, besides, I didn't think sumps were the way to go for a planted tank... But, I'm new to planted tanks. ;-)
Hope this helps!