I'd definitely go 12" for best results.
What do you want to grow? 8" is not a lot, but you could get away with it just fine for smaller veggies (peppers, maybe eggplant) herbs and lettuces.
You can leave the gaps as log as you pot something to keep the first from running out when you water. Just put a layer of landscape fabric, or even an old sheet or layer of newspaper on the bottom to keep the soil from running out. water will still drain, albeit slowly. If you have window screen, that works well too.
Do you need drainage on the other side? Maybe not. The wood isn't so tight that water can't drain. Throw a level on it, see which way the water will run.
If you want to to last, that's a different story. If it isn't lined that woods gonna rot in no time, maybe you get two years out of it. If I built that, I'd line it with heavy duty plastic and caulk the edges of whatever cuts or holes I made for drainage
