1- M2 SATA and NVM SATA are the newer card slots, which are smaller than SATA socket, just like PCI and PCI express ports. There are adapter available to used M2 and NVM with Sata, However ur laptop has SATA port, SATA SSD provided read write speed upto 6GB/sec
2-DVD also have similar SATA port but why u wanna use that for boot drive, u dont want to risk ur boot drive to get loose or connectivity issues. better install SSD in place of HDD and shift HDD in pace of DVD so that even if there is some loose connection issue with caddy and HDD still u will be able to boot up the computer normaly and diagnose issue easily.
3-RAM having few faulty registers normaly show such behavior, its good that u got a 8GB stick, addition 8GB ram will be cost beneficial and with SSD wil see RAM utilization upto 10GB with a few software running parallel.
4- Yes u need fresh windows on SSD as it wud have different file partitions and OS migration invites ininvited issues which later resolved only by fresh OS installation.
5- Yes the C drive will contain old copy windows and every bit of data till data, its ur choice to keep it as it is or format it to save up some space occupied by previous windows isntallation,
if u keep it intact, then in case u want to upgrade ru machine, u can simple unplug the SSD and put the HDD back in place and it will boot normally.
I would prefer an INTEL or SAMSUNG SSD with 100% health, please keep in mind that SSDs have lesser life as compared to HDD and its very hard to recover data from a faulty one, so better keep ur documents etc on HDD while being sued as data drive.
