Your biggest bang for buck is with cheap second hand drives, keep a spare on hand to rebuild the array / volume when one dies. You should be aware that the number of drives in the array directly affects the amount of usable space, 2 drives 50% of total available (a direct mirror, to compensate for the loss of one drive), 3 drives you get 66%, 5 gets you 80%. Say you get 6 4Tb drives, keep one as a spare and the remaining 5 will give you 16Tb usable (with one lost to parity so you can survive one disk failure). You then immediately want to save for a 16 Tb external drive for offline, preferably offsite backup (RAID is not Backup!). As others have wisely said, anything can be used to host, but aim at the most power efficient. If necessary get a PCI card for more SATA or SAS ports. Identify high value, small files, documents, current work, personal photos, source code and so forth and arrange for cloud backup, preferably with local encryption so you needn’t trust the cloud provider, preferably in at least two places (so one can go tits up or enshittify without bothering you). You’d be surprised what fits into a free 10Gb account if you triage well.

Good luck.