You should be able to find the info here:http://forum.wmdportal.com/toolpacks.php
It's a one-off purchase although chargeable DLC will be offered after release.
Hard to compare CARS and rFactor as they are different products in very different stages of development with different goals and aspirations. rFactor is a fully developed product with a a ton of content and has been around for ages with full multiplayer etc. CARS will knock you socks off but is currently pre alpha and is lacking in functionality and still has lots of issues that need to be worked out.
Right now i'd take rFactor any day of the week simply because i love the online community and the variety of racing it offers. but as i said, it's not a fair comparison really. If you can afford it, get both.
Comparing to rF2 could be a more interesting discussion though.....