The difference between HP iLO Free version and Advanced Version

This may be similar to my other question about specific use-cases of HP SIM at: https://serverfault.com/questions/284004/integrating-hp-systems-insight-manager-into-an-existing-environment

Either way, HP Systems Insight Manager is free and available from:
http://h18013.www1.hp.com/products/servers/management/hpsim/download.html

You can access ILO interfaces detected on your network from one place. HP SIM will also associate the ILO interfaces with the physical servers for you.

The software should probably run on its own Windows (or Linux) server, but for the size of the environment you're describing, it can run in a VM. Here are the minimum hardware specs from the Quickspecs page:

Any HP ProLiant with the following configuration:

Minimum: 1.5-GHz processor with 4GB RAM (for x86) / 6GBRAM (for x64)
1 GB free disk space recommended
Recommended: 2.4-GHz processor with 6GB RAM ( for x86) / 8GB RAM(for x64)

As for monitoring VMs, HP SIM is aware of VMWare ESX and ESXi and will display information about VMs discovered on your host servers. The HP-specific ESXi version will show hardware status and won't require any agents. They've already been integrated into the ESXi build.

**A note on the ESXi builds. I don't believe you'll get full hardware health info from any systems earlier than a G5. You won't be able to get ESXi 4 to run on the G3 servers (not 64-bit capable).

Pay for the vSphere Essentials package for $500-$600US. That will give you a proper vCenter, configurable with email alerts, for up to three host servers.

Short of that, you're limited to ensuring you have the ESXi CIM agents installed (or are using an HP-specific ESXi build). The hardware and storage health alerts will appear in your vSphere console.

See: Monitoring Dell/HP Servers Running ESXi (Free)

You can also view hardware status for all of those components using your ILO interface. An external monitoring package (HP SIM) is also helpful.