Selfhosted

I'd say that a good starting point would be the smallest setup that would serve a useful purpose. This is usually some sort of network storage, and it sounds this might be a good starting point for you as well. And then you can add on and refine your setup however you see fit, provided your hardware is up to it.

Speaking of hardware, while it's certainly possible to go all out with a rack-mounted purpose built 19" 4U server full of disks, the truth is that "any" machine will do. Servers generally don't require much (depending on use case, of course), and you can get away with a 2nd hand regular desktop machine. The only caveat here is that for your (percieved) use cases, you might want the ability to add a bunch of disks, so for now, just go for a simple setup with as many disk as you see fit, and then you can expand with a JBOD cabinet later.

Tying this storage together depends on your tastes, but it generally comes down to to schools of thought, both of which are valid:

• Hardware RAID. I think I'm one of the few fans of this, as it does offer some advantages over software RAID. I suspect that the ones who are against hardware RAID and call it unreliable have not been using proper RAID controllers. Proper RAID controllers with write cache are expensive, though.
• Software RAID. As above, except it's done via software instead (duh), hence the name. There are many ways to approach this, but personally I like ZFS - Set up multiple disks as a storage pool, and add more drives as needed. This works really well with JBOD cabinets. The downside to ZFS is that it can be quite hungry when it comes to RAM. Either way, keep in mind that RAID, software or hardware, is not a backup.

Source: Hardware RAID at work, software RAID at home.

Now that we've got storage addressed, let's look at specific services. The most basic use case is something like an NFS/SMB share that you can mount remotely. This allows you to archive a lot of the stuff you don't need live. Just keep in mind, an archive is not a backup!

And just to be clear: An archive is mainly a manner of offloading chunks of data you don't need accessible 100% of the time. For example older/completed projects, etc. An archive is well suited for storing on a large NAS, as you'll still have access to it if needed, but it's not something you need to spend disk space on on your daily driver. But an archive is not a backup, I cannot state this enough!

So, backups... well, this depends on how valuable your data is. A rule of thumb in a perfect world involves three copies: One online, one offline, and one offsite. This should keep your data safe in any reasonable contingency scenarious. Which of these you implement, and how, is entirely up to you. It all comes down to a cost/benefit equation. Sometimes keeping the rule of thumb active is simply not viable, if you have data in the petabytes. Ask me how I know.

But, to circle back on your immediate need, it sounds like you can start with something simple. Your storage requirement is pretty small, and adding some sort of hosting on top of that is pretty trivial. So I'd say that, as a starting point, any PC will do - just add a couple of harddrives to make sure you have enough for the forseeable future.