I would check out the E-Power 14650. It is an eGo type mod that uses non-proprietary batteries. The eGo is 3.4V. The E-Power and Riva are 3.7V. A 3.7V PV will allow you to use a greater array of attys/cartos like LR cartos which will give you a better throat hit and produce a warmer vapor.
Everything is replaceable on the E-Power also so it really keeps costs down. E-Power batteries cost $4 compared to eGo/Riva batteries that cost $14-20.
Read this about the E-Power :
http://www.e-cigarette-forum.com/forum/standard-e-cigarette-reviews/233335-anyone-considering-buying-e-power-14650-read.html
This will give you some inf why you might like the E-Power more than an eGo/Riva.
It will also tell you where to get it for the best deal, (Crystal Clear Vaping)
With the kit youll get a FREE 5-pack of "Ressurector" Cartomizers, a FREE 30ml juice of your choice, and FREE shipping.
Put "ecf5off" in and your get 5% off the order.
Its a really great kit to get you started. The Ressurector cartomizers are incredible and are awesome on the E-Power and youll get to try them for free which is nice. Youll get 2 batteries with the kit, so youll be set. I normally get about a day and a half out of a battery before I need to put a fresh one in.
As far as your other questions, I recommend you get some 12mg and some 24mg jui
ce and see what works for you. You want to make sure your using a high enough nic content so you dont crave a cig.
Check out BlueMist Vapor. They have a HUGE selection and really cheap prices. They have sample packs and I recommend you get a few so you can try a bunch of different juices to see what you like. They have a bunch of menthol juices.
As far as how long people use these for thats tough to answer becuase its a wide range. Many people enjoy vaping so much that it becomes a hobby. Use it as long as you need to keep you off cigs.
Goodluck with it! and let me know if you have any more questions