Most server hosts will give you plenty of hard drive space on your server since it is so cheap now a days. The other thing you might get if you have a premium account is cloud storage. This will allow you to store some data off of your server on the cloud which is great for redundancy. If your host does not provide cloud storage then an alternative is to get your own external hard drive.
If you have a server you should be doing back ups! I cannot stress that enough. The thing is most people back up their back ups to the server. This is great, but if the server hard drive crashes there goes your back up. I still recommend doing that, but take a copy of that back up and stick it on your external hard drive. No you have an extra back up in case your hard drive in your server crashes. Most people don’t do this until they experience a crash. I have experienced this and it sucks. Make sure you are backing up your servers data!