QUOTE
we have 6 Macs in the house in use at any one time, plus 3 external hard drives
Not to mention a few Teenagers!

I think DiskWarrier is like life insurance, it won't do you any good while you're alive. But if you don't have it your dependents may not keep your grave cleaned up! So far, I've never owned it. But I do have TechTool. Still, I've never had a problem that wasn't solved with other methods.
While I have OnyX, I tend to use it for easy access to functions that usually require Terminal. Putting double-arrows at both ends of scroll bars, showing the complete path to a file at the top of any Finder Window, anchoring the Dock to one end of the or the other at the bottom of the screen, add a message to the login window, make plain text the default for displaying messages in Mail, etc.
As you can see, most of these tasks are done once and fergitit! 
There are a few things that OnyX can't do that Tinkertool or Cocktail can, and vice versa. I can't remember if either of those are shareware but they shouldn't be that expensive, anyway. I like Cocktail because it can be scheduled to run certain tasks in addition to the ones that the System runs daily.
None of these apps will do anything terrible when you use them. They are much safer than using Terminal if you don't type well or know what most of the commands do.
Just enjoy your new machine! And keep those permissions repaired!