M.2 is newer type of slot which supports PCIE, SATA and USB interface (all M.2 Ports don't have all pins available), it isn't available on your laptop, on my last laptop it had M.2 SATA (no PCIE, NVME interface)
It's better if you use SSD in place of internal drive as it's your boot drive and you're less likely to run into OS corrupting/bios related boot issues.
Did you try cleaning your ram slot? And tried both ram slots with a known working stick? Maybe one slot is damaged. Upgrading ram is beneficial
I'd recommend fresh installation. It'll be much faster than installing windows on HDD.
You can copy your user folders (documents, desktop etc) to another partition and format the old C: drive and use that for storage. If you don't have storage space issues than leave it intact and you'd have option to dual boot from either SSD or HDD totally upto you.
Regarding brand, currently 90 percent Western Digital SSDs in market are fake Chinese with a slap on sticker.
Go for ADATA brand new for peace of mind. It isn't top grade but still it can saturate SATA III bus easily.
How much performance can you expect? My HP 840 G1 (4th gen core i7, 4GB ram) with all drivers installed took almost 5 minutes to boot up properly (background activity and disk usage settling below 10 percent), another minute or two to launch chrome (with 30-40 tabs was norm)
I installed a kingspec 128GB SSD and boot time went on par with my Macbook pro 13" 2016 to 8 seconds and chrome under 15 seconds. That SSD cost me 3600 from Aliexpress and i used it for 3 years before the laptop developed some other issues. The SSD was still working and the time savings I got out of it for 100rs a month. I can't use a PC without SSD anymore.
I also have a few Core 2 Duo desktops which are just as good for browsing with used but good health 128GB SSDs.
