Mac Pro 2013, also called Trash Can, was a deviation from the Apple Mac Pro line with its unique design. It supports up to 128GB of DDR3 PC3-1866 MHz ECC Registered Ram and has PCIe SSD flash storage. It only has one PCIe slot due to having only PCIe 2 x 2 lanes, so there was no point in adding another slot for a second PCIe SSD. For this reason, the maximum speed for PCIe SSD is around 1400MB/s read and 1500MB/s write (see the video below). Keep this in mind when upgrading it, as you do not need to spend more money on the high-end fast SSD if you cannot take advantage of its speed. The only exception is if you use this Mac professionally for photo/video/music editing because you need durability and high TBW (Terabytes Written, or TBW, is the total amount of terabytes of data that a Solid-State Drive (SSD) can write in its lifetime). Always remember back up your work, especially when you are professional.
For its lack of internal storage solution, Apple compensates by adding 6 Thunderbolt 2 ports to expand storage. Since this Mac will sit on your desk, connecting more external drives should be fine. One of the perfect external SSD solutions is Transcends 825 and 855 series external Thunderbolt 2 SSD. These were designed specifically for Macs with TB2 ports and are well-made and reliable.
As for the RAM upgrade, this Mac Pro was built with 16 or 32GB ram as 2 x 8GB or 2 x 16GB. It has four memory slots and can be expanded to 128GB as 4 x 32GB. However, you can keep your existing memory and only fill two empty memory slots with new ram, either 2 x 16GB or 2 x 32GB. There is no requirement for the ram to be the same capacity.
Internal PCIe SSD for this Mac Pro
Aura X2
Aura N2
LX300
Jetdrive 820 and 850 series
External SSD
Jetdrive 825 and 855 series
Ram upgrade