I may be wrong, but I think you might be taking about an Amazon Kindle with the 3G access. All of the Kindles have Wi-Fi, but if you need independent Internet access all the time (as long as there is service) only the one with the 3G connection will do.
The original Kindle (I think I bought my first one in 2008) came with the cellular connection included at no extra cost. Since then, it is an option for which you must pay extra. I'm currently in my 8th Kindle, a Paperwhite with the 3G. I bought this one with the 3G, because I knew I would be on the road and I NEED MY BOOKS WHEN I NEED THEM!
You can use the Kindle for their clumsy email service, and "experimental" internet connectivity, but you will pay a nickel or dime per their proprietary email (I can't remember exactly). I've not used that feature. The Internet access on the Kindle is slow and sad at best. I only use it to download books, but because I read a book or two a day, I use that feature all the time. I think Amazon Kindle uses the AT&T network.
The current Kindle Fires come with the Alexa feature, but you need Wi-Fi to use it.
When you buy a Kindle with 3G, you only pay for it the one time, when you purchase it. Keep in mind, though, that if you use the Kindles hard, they only last a year at most (at least for me). The screens just go bad with no warning. Like I said, I'm on my 8th Kindle, only one was a Fire. It worked still, last time I saw it. I'll find it with emptying the house for sale, but the interface sucks. I'd much rather use my non-Kindle tablet for everything, including the Prime video access. You can download a bunch of movies for free and watch them offline.
And, if you have any subscriptions, you cannot use them with the Kindle unless you buy them separately. For example, I subscribe to the Economist and the New York Times. But if I want them on my Paperwhite, I have to buy the specific Kindle subscription. Thankfully, as long as I have internet access via Wi-Fi, I can download all the weekly issues of the Econmist on my Lenovo tablet. The NYT I only use while I have immediate Wi-Fi and I'm surfing.
I don't know if I've muddied the waters or helped clear them up for you. Whatever options you use, its gonna cost you. Your best bet for Internet access cheap is at public libraries or stores/coffee shops/restaurants. As you go along in your vandwelling journey, you'll be better able to accertain what you need vs what you want vs what you can afford.
Ted
