1. Can you afford 64GB? Then get 64GB.
2. Open the bottom cover of the Mini 9 - remove the two screws first. With the battery "up", look to see what's inside. Upper left is the SSD
. Upper right is RAM, lower right is WLAN. Do you have a card in the lower left space, or is it empty? That's where the WWAN option goes. Unless you ordered the 9 with the WWAN, it won't be there - not even a socket to go into (usually.)
3. SLC is faster but MUCH more expensive, and certainly not worth it on a 9. Go for MLC.
Welcome to the forum!