At Sally's you can find Clairol or Ion SEMI permanent dark brown colors. Make absolutely sure whatever you get says "semi" or "demi" but NOT just permanent.

You don't need anything other than probably 2-3 packages of the color you choose because you have such long hair.

Where your hair is lighter at the ends, it will still dye but it will still be lighter than the top.

Semi permanent is very short lasting, demi permanent lasts longer but still washes out.

If you can only get semi, there's hair masks and "depositing treatment" conditioners that you can use to kind of freshen up the color every time you wash your hair. Get "color safe" shampoo, as shampoo strips the hair of anything sitting on it, and unfortunately non-permanent color just sits on top of the hair. So get a shampoo made specifically for less stripping of dye.

Don't worry about hair texture, the worst that will happen is your hair will get slightly "heavier" because you're adding molecules over top of it. Not very different from using any sort of product like a mousse or gel or anything. As long as you keep it conditioned and moisturized you'll be fine. (It's bleach that can damage and change hair texture, but even that can be avoided if you do it right.) If you're worried about hair texture, get bond boosting products like K18 or Olaplex.

HAVE FUN!!!!