The reliability of SSD cheap or otherwise vs. spinning rust (conventional hard disk) is irrelevant, neither can be guaranteed not to fail, so the relative MTBF (mean time between failure) comparison is pointless. You have to backup your drives, an inexpensive portable USB3 disk or similar is perfect for backup. Even if you have RAID 1 or 1+0 or 5 etc or even triple mirror. You still need a backup. You need to protect from Logical failure (failure caused by you, your software or the Operating system) as well as physical (device failure).
Tony - your machine built for XP, which is now too old or slow to run windows 10 etc.. may run Linux min 18 - project Sarah very well.
https://www.linuxmint.com/download.php its an open source (free ) alternative to Windows based on the Linux platform (the no1 OS for enterprise servers )
You can get 120gb SSD for <£40 which will be 10 times faster than a typical 7200 rpm drive. I'm currently running the above on an old DELL i5 laptop.
It boots in under 20 seconds. running windows 7 it was taking about 1 minute.
it is very hard to catch a virus, because its linux not windows.
its very similar to a mash up of windows XP, 7 and Mac OS in look and feel
It has lots of great software for free (equiv. of word, excel powerpoint, photoshop, imovie etc ) are pre-installed and free.
It will play any video format, not just those approved by Microsoft.
I'd seriously consider trying it in your current machine and forget the whole windows key thing until you have.
In terms of hard disk - its really size vs speed. a 1TB disk is about the same price as 120GB SSD. So how much do you need. With desktops you also have the option of hybrid - OS and programs on SSD and data, pictures etc on HDD (rust).