Mac OS X is safe to use. It's entirely safe to remove IE; all you have to do is slide the folder in the Trash. No more MessIE.
The kernel is audited by the community, and therefore is just as safe as other open source OSes. And Apple has a good reputation of fixing bugs as soon as possible. There has been no case of Apple sending private information, and the possibility that they may do so is highly unprobable. Again, their reputation of respecting privacy is spotless.
From using Macs at school, and comparing it to Windows, I can sincerely say it is a very legitimate alternative, and much easier and more stable. In my case, I always found what I needed in Mac OS X without resorting to MS software. You can even use many open source software like OpenOffice and Mozilla.
I strongly recommend you to buy your Powerbook, if that's what you want. I myself intend to switch eventually.