Hi Ray, and welcome to ECF!
Liquid use varies quite a bit. No real "how much is equal to..." So figure that on the high side, as you don't want to run out! 3 ml per day isn't uncommon. But you'll want to be trying different flavors, so the amount on hand will partly depend on how many flavors!
The MVP2 is certainly a good device, but the box shape isn't everyone's favorite. Its little brother, the Itazte VV3, is another good choice, with all the same features but only 800 mAh, versus the MVPs 2600. So run time between charges is a lot less.
Is vaping cheaper? Obviously, in the long run, yes. Cancer treatment is very spendy, in all kinds of way!
But initially, it may not be. Initial hardware costs will run it higher, but as long as you don't let us get you on the "OOOOH! Bright and Shiny gotta have it" bandwagon, those costs will settle down.
My neck of the country, my initial costs for a starter kit, with a couple bottles, was about the same as a carton of cigs. And lasted a lot longer!
But that said, you do want at least two of everything. Backups are important, as you can rarely predict when hardware is going to fail.
Yes, the tank is where the juice goes, and having multiple ones allows you to have several flavors ready. Sometimes its nice to switch off, and if you only have one topper it's a hassle. (Empty the topper, clean, dry, you get the picture!)