My two cents. If you don't plan on accessing the data anytime soon, as 2blackbar suggested, LTO tape drive is the cheapest and safest solution.
Even tho a lot have been done concerning data integrity with overprovisioning, I wouldn't trust SSDs for archival. Flash charges dissipate and data recovery might be problematic because of proprietary firmwares, controllers, etc.
I'm cheap, I'd go for like 4 SAS hard drive in raid 10. I would mix brands (at least not the same batch) and go for slower 7200rpm drives.
I'd get the best PSU available for my budget (platinum/titanium like, anything more than 90% efficient at 100% load).
I'd get a reliable, brand new, consumer grade motherboard, a cheapo cpu, few gb of ram and an SAS HBA card which is quiet expensive and not always linux friendly (google-fu required).
Everything on a DIY clean and well ventilated cabinet install debian and barely touch it.
I think SAS drives are generally 10% more expensive but they are more resistant to vibrations. It's server grade, MTBF of SAS drives are 1.2 to 1.6 million hours of use at 45 °C and their SATA counterpart only 700,000 hours to 1.2 million hours of use at 25 °C.
For recovery, Ontrack seems to be a serious international company.
https://www.ontrack.com/uk/services/data-recovery/hard-drive-recovery/edit:
There's also cloud solutions, you can encrypt everything if needed.
LTO tape is cheap if it's several TB per months of archival otherwise it's not worth it. You can find fast LTO7 SAS drives (at least 300 Mbps) for something around £2,000 on ebay. A new 15 TB RW cartridge (30/45 TB of deduped/compressed storage) is around £60 .
If you take the hard drive route, there's also a raid backup open source software called SnapRAID.