Need guidance, storage purchase imminent, limited budget. Recert / Refurb Drives

I need to reach a certain amount of space but have a limited budget. I saw these refurb drives on bargainhardware a while back and waited through BF to see if anything close to the £/TB came up. Unfortunately the UK doesn't seem to have seen any such deals.

My budget is limited, I also already have some older smaller drives that would form part of the array. I would likely be using unRAID with 2 of the purchased drives being Parity drives, or the same with SnapRAID. I feel that 2x Parity is neccessary to ensure that I can protect against inevitable failure, my current 4TB drives approach 5 years in use, and the refurb drives are between 3 and 5 years old according to the seller (datacentre use). If you feel you can suggest better alternatives or new drives that fit the bill please let me know. I feel that I would be unlikely to return a storage drive under warranty.

• 4x 10TB HGST HUH721010ALN600 £86.40 - £345.60. 20TB pool, approx 5 years usage.
• 3x 12TB WD HC520 HUH721212ALE601 £132 - £396. 12TB pool (higher parity ceiling), approx 3 years usage.
• 3x 12TB EMC Seagate ST12000NM005G £120 - £360. 12TB pool (higher parity ceiling), unknown time in use, Seagate?

I don't want to go above £400, I have a need for at least 10TB in the pool and in my mind 2 Parity drives is mitigating some of the risk with refurb drives. Am I going about this the wrong way?