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.
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.
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.
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.