Regarding longevity of Apple kit, I have a 27” imac with the 5k retina screen that i bought in 2014. It has been working perfectly until last week when it became unbearably slow to boot and then impossible to boot. I was reluctant to bin the imac with its beautiful 27” screen, especially as the new imacs are only 24”.
A quick bit of reading around suggested that the fusion drive was on it’s way out and it would be possible to use a bootable external ssd drive as a replacement. So I bought a 2TB external USB SSD drive, and followed the instructions I read on the web (boot the imac into the recovery mode, format the drive, downloaded a copy of the OS onto the new drive and set it to boot from that drive). Dead easy.
I even managed to copy the all of the data and settings from the dodgy fusion drive onto the new drive so it is set up exactly the same as before.
Hopefully this should see me through a few more years as my use pattern is very non demanding ( web surfing, document generation, music streaming, but no video editing or gaming).