It's only a half step below standard, so I'd say gauge 10s are good enough already. In fact, gauge 9s would work, too, but for more solid tone and feel, perhaps 10s would be better.
I used to use gauge 10s in D standard tuning (DGCFAD). It worked really well and felt really comfortable.
Brand-wise, it's subjective. You can use the same strings for standard, or down-tuned. Just that if you use really low tunings (like C tuning or even lower) , you need a thicker gauge so the strings don't feel floppy. Brands-wise and tone-wise, it's subjective. I personally favour Ernie Balls.
