Maybe take a look at a Velocity clone. V1 or V2 are really good to learn with and easy to build on.
fasttech has a V3 but I personally don't like it. I use my V2 on a Kanger Dripbox 160 everyday and have tried and own many other rda's. I spent $30 on a Tsunami that sits in a box. Stick to a velocity style build deck and see what's being used at your local B&M. The best advice is to watch reviews.