You do not need 8 GB of RAM ......
Every computer that I upgraded had a SATA HDD in it.
I simply do not believe Win 10 is lighter unless you unpack a bunch of features and apps like Cortana as you have done.
I think the key to your post is SSDs (which are a great upgrade for any computer).
When an operating system runs out of ram, it doesn't crash, it creates virtual memory (swap file) on the hard drive. Both Windows and Mac do this. The problem is, a hard drive is about 100 times slower than ram, so if you have a lot of swapping going on with frequently used parts of the OS, you'll hear your hard drive thrashing and your computer will perform like a slug.
An SSD will typically be 10-20 times faster than a hard drive, still slower than memory but the effect of the swap file is much less pronounced. But using an SSD as a substitute for ram is not optimal in the long term since SSDs are not designed for huge numbers of writes and writes are relatively slow.
Really, given that SSDs are so cheap, I'd do an SSD upgrade before I'd upgrade ram. 128 GB SSDs are like $40 now and 250s are $60-70.
But ram upgrade is cheap too. Upgrading ram from 4 to 8 GB is typically only like $30.
So if you have a computer from the last 4-5 years, you can dramatically improve performance in ANY OS by spending about $100 on SSD and ram. they're both relatively easy to do yourself -- the trickiest thing is cloning your HD to the SSD. (and BTW you don't have to invest a lot in special brackets to mount the SSD in a desktop computer -- just use Velcro tape because SSDs are super light.
The smart way to run a computer is to back up your files on the cloud or a server in a different location. Keep your local copy of bulky files (video, music, photos) on an external drive (but always maintain at least two separate copies, and important things should be cloud backup).
If you do that, you probably will only need a 128G SSD. Though the 250G ones are so cheap, $60-$90, I'd probably buy that size now.
SSD brands do matter. Pay attention to the Amazon and Newegg reviews. Samsung SSDs are generally recognized as the best but they're 40-50% more expensive than other brands. Kingston, Crucial, Sandisk, Adata and PNY are all selling a ton of them and doing a good job. Adata and the Kingston SSDnow line are super cheap and extremely popular.
I think SSD failure rates -- even for the cheapest brands -- are already lower than HD failure rates. But obviously, back up or build yourself a RAID.
You do not need 8 GB of RAM or even a processor that is within the last 5 years such as an i5 processor. Every computer that I have installed Win 10 on ran better than before upgrade. Win 10 is or can be a lighter OS than some previous Windows OS. I have installed it on 2 Dells at work that came with XP and are maxed out on RAM at 2 GB that are about 10 years old. Before XP End of Life, I upgraded them to Vista because I did not have enough Windows 7 licenses laying around. The 2 Dells ran like a dog with Vista with almost everything killed to run the best it could and used the speed boost feature using an USB thumb drive. I installed Windows 7 on them with the intent to upgrade to Windows 10. Windows 7 ran better than Vista, which I expected. Windows 10 runs better on these 2 Dells than XP, Vista, and Windows 7 without using the USB drive speed boost feature. I have always killed Cortana. Every computer that I upgraded had a SATA HDD in it. I have to upgrade a HP computer or 2 in the office that has more RAM than the Dells but are EIDE/PATA HDDs. It is possible that when I upgrade these 2 11+ yrs old machines that I will find these former XP boxes run like dogs because of the hard drive difference.