https://www.backblaze.com/blog/how-reliable-are-ssds/
Not a bad read about ssd vs hhd. They're a data storage and backup company, and this is written in 2019, and already they're starting to use SSD's in their arrays.
Personally, I think it's all a bit of luck, but hhd fail like everything else. My first built pc had a hhd which was as good as gold. Had a laptop 10 yrs where I used to transfer files onto a portable hhd. Unfortunately, my toddler ran passed and managed to snag the power cable and pull it off the table. Hit floor and that was it. Had a lot of photo's stored on it, so we sent it off to a specialist, who tried to rebuild it using a second identical hhd, but data was too corrupted.
Then went down imac route, hhd was replaced under warranty, and new one failed 2 years ago. Just refused to boot up. And laptop I'm on now takes forever to do something, and makes a clicking sound sometimes which continues when booted up. Task manager showing hhd at 100% all the time, so think this one may be on the way out too.
Think people still use hhd for cost reasons. They're still a lot cheaper than ssd for large storage. If you really want large ammounts of storage have you thought of a seperate NAS..get a 6 bay one, populate it in a raidarray so has copies to cover failure of a drive?
Build i put for you has 7tb, 1tb pcie4 primary which is fast(faster than the firecuda 520) and comes with 5yrs warranty as opposed to 3yrs. Firecuda 530 comes with 5 yrs, but costs £189, so a touch more than the SN850. Secondary 2tb is the SN550. Not nearly as fast compared to the SN850. It's pcie 3, still fast but put in for cost as basically £190 for the 2tb. Step up would be SN750 at £290 , but seagate 2tb 510 on offer at £270, so just pips it for price. If you want pcie gen 4 your looking at the Firecuda 520 at £380, then Firecuda 530 at £410, and SN850 at £430
You could have course just get OCuk to buid with just the primary m.2 ...I managed to get the 2tbSN850 for £280 which makes it a little more palatable than £430
The sata drive was 4tb, and comes with 3yrs warranty. you could drop it to a pair of 2tb drives. The WD drives come with 5yrs warranty I believe, but hava check. Also ask OCuk what they'd recommend to, seeing as they're doing the build for you.
Cooler I put it to keep a theme. It had good reviews, but EsaT is right, the Arctic freezer 2 is cheaper, and has excellent reviews(generally top pick)
Thinking about case, if you really wanted hhd in there too, you could go for something like the Lian Li Dynamic Xl. The case is £205 though, but comes with a hot swapped caddy for hhd, and also a place to put your sata also. Loads of space in there. you'd need to get fans as comes with none . The new lian li uni sl120 or al120 would be good. They produce good airflow and are daisychainable so cable management will become a lot easier. I'd do intake bottom and side, exhaust at top with rad, and 1 at back also exhaust. 6 in/4 out to keep positive pressure. Seems to run coolest that way from review setups, though some put rad on side with a push/pull config
The corsair 5000x great if doing just m.2 and sata, but the hhd cage can get in the way a bit with all the cabling. Also at £180 odd, OCuk charging a bit. they had it at £140 for a long time, and I picked mine up for £120. So as you're doing a big budget build, you might want to negotiate a little bit
cases all subjective and there are a few really good one out there
And the Ram, only 1 stick of the sammy b die (8Pack) left in stock at mo if you go that route. The crucial is next best...the corsair you had at £219 or whatever you can buy elsewhere for £150 odd. EsaT listed them in last post
Good luck with your decision, let us know what you're going for. always interesting to see