Yep, it's a bit confusing, my advice is to delete mods and things you don't need atm, eventually a new fresh rF installation is your best choice, then backup your data.
Once you are ready you can try all the mods you want. Without to worry about if something doesn't work, you can always restore your backup. That endruance mod need the DX9 libraries, if I'm right the Porsche works only with DX9.
They have different versions of the download, a normal one and a splitted one. They also have a huge number of mirrors in case a link wont work.
You should download the "Endurance Series - v1 + SP1 FULL (including v1.0 + v1.2 or v1.2FULL)"
(or if you prefer the splitted one)
and then "Endurance Series - Service Pack 1 - PATCH (v1.0 + v1.2 or v1.2FULL needed)"
(there is also a splitted version for the patch if you want)
I'm not using rF a lot in these days (I'm running the WSGT mod for GTR2 there should be a rF version somewhere) but If I remember right there is a tool somewhere at rF central that allow you to switch between the mods.. without the need of uninstall them. Still have to try it by the way.
You can also try Bigbazz's advices. rFactor have lot of mods it depends on the cars you want to drive.
