Browse through the threads on Stevia. Lots of information!

http://forums.gardenweb.com/forums/load/herbs/msg0815435324636.html

Here is a link that might be useful: stevia

In zone 7, maybe you should hold on planting now.
I would plant it when I am planting my peppers and eggplants.

Give it a lot of space. I would say a 2' by 2'. Put a stake next to it at the time of planting or use a small tomato cage. It can grow over 2 feet tall and branch out, bushy. Branches will fall down if not supported.

I think they like well drained soil. I have had some that died in wet soggy condition.