Hey, you ask a lot of question who already have their answer in the forum.
Vultr
Any VPS is good to install Flarum. Note that you'll need :
As such their first offer 20GB SSD is ? .
Install
In my opinion (and because i'm a Debian enthusiast) Ubuntu is too much for a webserver but if that's your choice, go with it. To install Flarum you will have to follow theses instructions : http://flarum.org/docs/installation/
Ask !
if you need anything else, ask ! We're not in your head ?