1) Wanted to check which all AEM Health check tags we can trigger per minute or per hour without adding load on AEM. We are interested to know CPU Load , Memory Usage , Disk Usage frequently , Indexing Job - so that we can be alerted if there is any Spike. But wondering if we configure these alerts per minute or per hour will this add any load to AEM itself. Should we worry about this.
2) Also noticed that Adobe OOB had Health Check Tag (repository , disk) for capturing DISK USAGE but do not see any Health Check tag OOB for Memory Usage or CPU Usage - Is this done purposefully as these (checking CPU and Memory) might add load on AEM ?
You need to distinguish between system monitoring and AEM monitoring. For system monitoring (CPU usage, disk usage, available RAM etc) there are dedicated tools which can do that much better than AEM itself.
And even for AEM itself I would use a dedicated monitoring system which is able to fetch data from the AEM healthcheck servlet. Personally I would not use the HealthCheck emailer, as emails are quite cumbersome to parse, and monitoring is something you definitely want to automate.