My aim is for 4x10TB to run on RAID 5.

I strongly recommend not putting 5x 10tbs in raid 5 if you value data integrity of the set. Chance of failure during a rebuild following a dead disk is disproportionately high.

Consider instead: Raid 1+0, Raid 6 or ZFS. I use 1+0 in my 4x4tb set as a reasonable compromise for usable space and speed.

One of thousands of such articles; https://www.askdbmgt.com/why-raid5-should-be-avoided-at-all-costs.html

I wanted to reply that snapraid or the equivalent will lead to less wear and tear, and provide reasonable redundancy that you can trigger as needed. It's also a lower electricity bill and allows mismatched drive sizes.

It's also arguably easier to recover data on failures.

Can't help with the drive costs unfortunately.

TBH if you're OK with 12TB then shucking drives can be a good deal for TB/pound. 12TB usually guarantees the IronWolf/Exos/Red Pro drives

SAS drives are much cheaper if you're fine with getting a controller card

I’ve used Robert’s Electronics several times in the past. Generally ship from Singapore but never had a problem with them. Always arrive well packed and have always been new and sealed.