Most questions will be covered by the following articles or posts. ARC is ZFS just using your RAM. L2ARC is for reads and can be added, if necessary, after your system is running for a while and you have a proven need. SLOG is for sync writes, so not needed unless your system is doing those.
BASICS
iX Systems pool layout whitepaper
Special VDEV (sVDEV) Planning, Sizing, and Considerations
