Syncthing runs on my raspberry pi (model b with 512MB RAM) which is pretty low end 
for the RAM usage there is a example benchmark here: Initial v0.9 benchmarks which shows ~100MB needed
most systems should not have a problem with that
CPU for my raspberry pi is at 100% while syncing at ~1-2Mbit which is the limiting factor here (still using 0.8.21, i have no info about 0.9 which could be faster without compression). Idle is <1%. Rescanning files also needs CPU but the time until it does that again can be set to a high value so this is also no problem.
Since only the sync speed really depends on the CPU this is something that depends on the user how fast he wants it, there is no real requirement i think.