A:AnswerYes and No. First off I wouldn't have tried Fortnite if you didn't ask.
Yes: 1080p epic settings 60fps. Lag issues when background programs are running. (was downloading something on steam) But that could probably be fixed with more ram.
No: It was all white. Couldn't see anything. Did some research. Found out I needed to update the drivers. I still get this message but I just updated to the recomended version and it fixed the issue.
Verdict: It can run Fortnite at 1080p 60fps max settings with 0 lag.
A:Answer2400mhz, and you'll need to stick to that when upgrading.
If you upgrade, get G.SKILL FORTIS Series 16GB (2 x 8GB) 288-Pin DDR4 SDRAM DDR4 2400 (PC4 19200) AMD X370 / B350 / A320 Memory (Desktop Memory) Model F4-2400C15D-16GFT. It's virtually guaranteed to work and is made specifically for Ryzen systems.