A:Answer This desktop has Intel Optane Memory, which works like an SSD Cache. If you don't mind performance roughly equivalent to an SSHD (Solid State Hybrid Drive), where the startup times and your most used programs will be accelerated to around SSD-speeds, then you don't even need to add an extra SSD. Since Optane tends to come in 16gb and 32gb configurations though, it'll be very likely that this computer would have performance actually superior to most SSHD's, and for most things you do, you'll get very close to SSD performance.
That said, since I don't own this computer, I couldn't tell you what kind of SSD to get. If the mobo has an M.2 slot, I would guess it's taken by the Optane memory though.