I'd recommend building your own server rather than buying an off-the-shelf NAS. The NAS will have limited upgrade options - usually, if you want to make it more powerful in the future, you'll have to buy a new one. If you build your own, you can freely upgrade it in the future - add more memory (RAM), make it faster by replacing the CPU with a better one, etc.
If you want a small one, the Asus Prime AP201 is a pretty nice (and affordable!) case.