Hello,
I hope you enjoy your new venture into 3D and Blender, it’s going to be a lot of fun!
Just to answer your questions,
Any 3 button mouse will do but, as always its worth spending a bit of money on a half decent one. You really do want one with a scroll wheel too as that always come on hand using 3d software. Having said that I’m assuming you have the Magic Mouse that comes with the iMac at the moment. Although not the best for 3d work you can use an application called MagicPrefs that will allow you to enable middle clicking turning your Magic Mouse into a 3 button mouse!!
As for Blender being safe to use, you have nothing to worry about. It won’t harm your mac in anyway if used properly. Always download you copy from blender.org as this should ensure your copy is good. There have been “paid” versions of blender kicking around the web in the last year but, these are not to be trusted. Blender is a free product and should not be paid for.
On a final note, I’m sorry if this is something you already know, your mac is not immune to malware and you should always be vigilant and take all necessary precautions to protect yourself and your mac. Download from trusted sites, keep your mac up to date with the latest patches and really you should install some sort of antivirus software.
Anyway, enjoy Blender and have fun!