You can actually go through all the steps, right up to the last bit about the .htaccess file. Until you do that step, none of your traffic will be FORCED to use HTTPS. Regular HTTP traffic will work as normal.
So that gives you time to test it out. See if you have any images that are not secured. You basically looked for "mixed" traffic. Something on your site that isn't secured and should be. There are sites out there that you can use as tools to inspect your pages and find out what isn't secured.
The "site certificate" will need to be updated every three months. So the part where you download a new ZIP file, install the certificate.crt and private.key files, will need to be done every three months.
Will the world end without HTTPS? No, not really. Chrome will probably complain. But since most of our content isn't delivering files or other things that can infect a computer, it's not a real threat. But knowing Google, they'll probably push the warnings at some point stating your site is a "potential threat" or something named close to that. My guess, you'll have a couple of years before it gets to that point. Maybe longer.
If you buy the Google one, I would think they'll be doing the main work and installing it on your site. But I have no first hand knowledge of that. Worst case, the cert you buy will be good for what ever years you pay for. You may have to go through those steps to install it. But it can be good for years worth. At that point, it's more a matter of what you are willing to pay.
If you are thinking of buying a cert, you can shop around for that, just as you would have for a website hosting provider. I think we'll eventually get to the point where hosting companies will start providing them at low or even no cost. Some hosting providers are setting up a way to get the Let's Encrypt certs automatically, so you don't have to go through the hoops I outlined. But of course GoDaddy wants to charge you for certs, so they have no incentive to get that going. Not until the competitive eventually forces them to.