Heroku can (and will) answer those questions for you. Submit them as "pre-sales" questions and they'll tell you the facts.
Last time I asked (for myself), they said that traffic is unlimited, you pay only if you increase your dynos to 2 or have any add-on features. You should note that Heroku is not designed to primarily serve up static assets (Read more on their website about "slug size" and hosting static assets on cloud-based storage for an explanation.)
Heroku is ideal for Rails apps you want to get off the ground quickly. You can always throw it up and decide to move it later. You'll probably learn a lot about Heroku and building a simple Rails app by doing so, so it can't hurt to try.
As far as "is it free for a lifetime" the short answer is you have to ask Heroku, but in life there are no guarantees. Their current offering is that the first dyno is free, all additional dynos and additional services you pay for. Read more about what dynos and workers are for an understanding of what this means. That's their current pricing model but of course I'm affiliated with nor am I speaking on behalf of Heroku, just telling you what I know.