There are free solution like Cacti or Nagios which can provide all you have mentioned for free.
Personaly i use Cacti, i have been able to demonstrate that it is working with all stuff required (alerting, capacity planing, equipment support).
It is also completely open source, so you can hack around it (it is php / mysql / apache).
A shell script based solution will be hard to implement to work on all unix/linux operating systems (it can be done, but it will require much more effort).
It will be a monster script at the end if you wish to support everything
That is why folks use SNMP and/or agent based monitoring with central system.