Hello and welcome to the forum!
I'd love to give you definitive answers for your questions, however, every vaper is different so it's not possible to say what your vaping habits will be.
The average liquid consumption seems to be 3-5ml/day. Some people vape way less than they smoked and others vape way more so a comparison to packs/day is iffy.
Vaping can really be as cheap or as expensive as you make it to be. If you find a good unit (the Riva is a good choice BTW) at a good price that works for you, your consumables and liquid costs are nominal. The biggest expense that most new vapers see if through the trial and error process of finding a good setup for them as well as trying tons of liquids to find what they like. Once you get through the learning curve, the costs level out considerably. I splurged a bit with my tax refund this year and went a bit crazy, but for the past year I've averaged about $200/yr on supplies for 2 of us where our smoking habit was $400/mo.
Now if you get bitten by the "mod bug" (needing to have every new shiny thing that comes up) you'll blow through funds in a heartbeat lol
Atomizer life is hit and miss, they can last a couple days to a couple months. I wish I had better news on that, but that's just how it is. I would suggest trying a couple different types of atomizers as well as some cartomizers to see what you like best and then stock up as backups are always a good idea.
The Riva 510 is a good start as I mentioned. It can be used with a large variety of other 510 threaded components which will give you plenty of variety as well as vendor options.
Best of luck to you and I hope you'll love vaping as much as we do
