what kind of SD card?
All modern everdrives should be MicroSD (the tiny ones), but always check the specifications to be sure.
Perhaps sending an amazon link would help.
This is the one I use:
https://www.amazon.com/dp/B073K14CVB - 16GB SanDisk. A full N64 set is about 4GB. Maybe get the 32GB one and call it a day.
what are your favorite features or unique games?
The two I use the most are APS patches (removes software anti-aliasing [makes image a tad sharper]) and creating Gameshark cheat files for my favorite games.
No, the R4 will act as your NDS card. There will be a slot for a micro SD card in the R4 which will act as your storage.
The Super Everdrive X5 supports up to 128gb SD cards. You need a formatting tool to get anything larger than 32gb to work.
That being said, Amazon does sell 16gb to 128gb of Sandisk MicroSD cards.
https://www.amazon.com/SanDisk-128GB-microSDXC-Memory-Adapter/dp/B073K14CVB
Here is the ethernet splitter. I added this to Godzilla, then ran a cable to my other pin. Note that after setting up connected and home team on the other pin I do not leave this connected. You only need to have it connected when you scan and register the machine the first time.
https://www.amazon.com/gp/product/B00WTRWPU0/
Here is the SD card I added: https://www.amazon.com/gp/product/B073K14CVB/
(This is a blank card you add in the back box)
Here is the wireless card I used: https://www.amazon.com/gp/product/B00H28H8DU/