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Server status page that updates every 5 minutes and displays:
Linux server load (uptime command output)
Disk Space (df -m output)
Email queue counts (exim command that tells how many emails (counts) in queue)
Service status (https, mysql etc status - red or green image)
Logic is you need to write each server output in a txt file via shell script and then a monitoring server just pick-up those values via 5 minutes cron. And a simple no-fussy admin section that allows to add/delete servers. (I am having one old script which do a part of above which you may improvewise)