They don’t sell them on their own website anymore. You can buy them only on stacksocial. Site looks a bit shady, but it is actually legit and in partnership with koofr. VAT is excluded from their payment screen, so do take that in mind. Later on you can upgrade your account to 2.5TB for an additional 300€ directly on koofrs website.
To protect your privacy, always encrypt your sensitive uploads with rclone or something similar. The backend is almost irrelevant then. Still, there are often some unforeseen bugs or artifical limits set by the providers. The only way to find out if a service really works for you is stresstesting it with your workflow. Most services offer a few GB for free to do that. Some examples of what may go wrong: A few years ago I’ve run into some problems with Proton and Zotero and quickly decided against it. My KeePass app stopped syncing with a seadrive server at some point. One nextcloud provider didn’t like my seedvault backups and always dropped them halfway through.
I’ve been mostly using Koofr because it’s european and there is a lifetime 1TB purchase available on stacksocial. Some view lifetime subscriptions as a controversial business model, but I like it better than a monthly subscription. Koofr also encourages rclone usage and I haven’t run into any problems yet, but, as stated above, your mileage may vary.
To make a fresh install less painful: Most applications save all user data in your home folder under .config and .local. Some others make their own .folders, which you can see pressing ctrl+h (for hidden) in your file explorer. You might save these and carry them over whenever you wipe your OS. If you save a list of installed packages from your package manager as well, you can get up and running in just a few minutes on a fresh install.