Hi dsmodi,
It will be fine to install on a PC already using IE6 - you can even install the new version of Netscape 7 if you wish. As for cost, there is a free version, but if you dont want the ads, you';; have to pay for it. The only other conceivable problem is that a lot of web developers don't check a browser like Opera to see if there pages look correct (myself included) so you're more likely to come across website that don't look as good as in IE.
Cheers
Paul.