Ideally one that can use more than one disk so that i can expand it later when i can. Have some minimal experience with Synology since there's one at work and i have interacted with it a couple times and like the interface, but am not married to any brand as long as it works.
I made one with an R-pi and hard drive with Open Media Vault. It was pretty easy to set up, and I haven't had any trouble with it. Only one disk so far, since I don't have that much stuff.
I wouldn't recommend a Pi 4 even if you can get one at the listed price. It is fast enough but it lacks the interface and connectivity for a decent NAS.
The main issue is if you try to attach multiple portable drives. My Pi 3B+ could only really power a single HDD, but I wanted to attach multiple. The only solution there was a powered USB hub. I eventually got an old Lenovo mini PC and used that.