Sounds like your mind is made up.
One additional thing I can suggest would be the Munin monitoring software, which you can install through WHM... it may be just what you're looking for, without the $300 price tag.
I still think that "Lithium" is overkill for you (it seems to be aimed at more of an enterprise level, multi-server management setting) but if you can justify the $300 payment, go for it.