Honestly I've heard a lot of issues are with the M.2 SSD, some of them are issues of the units DOA or even installations of Windows 7 or Windows 10. If your able to increase your budget a few more dollars I saw recently 480GB SSD for 112$ @ Bestbuy.
However, M.2 is a small form factor and can have higher read and write speeds depending on your needs, Honestly personally, I don't notice a huge difference from day to day use and loading games being at 400/s read & write or 500/s.
For your memory question that is totally up to you, depending on your needs. If your gaming most games don't require anymore than 8GB. I honestly only would upgrade to 16GB if you know in the future your going to be doing sort of video editing or running virtual machines.
In that case save some money on the ram by going with 8GB and than going with a larger SSD.
Good Luck!