I can't speak to Win10, but I've been using Windows 8.1 64 Pro without issue. I'm not hesitant to upgrade to Win 10 either, I suspect it should be fine. Hopefully someone else can chime in who's upgraded already.
The go to mod is Rallyesim's RSRBR mod, which includes a whole ton of cars and tracks. It's what we use online here for the club and RDRC rallyes.
In the default game there are 8 cars I believe, WRC cars from the early 2000s. There are 6 countries, 1 tarmac (France), 1 snow (Finland), and the rest gravel (Australia, USA, Japan, UK). Each of which has four or five stages + reverse. Not a super amount of content, but with mods it increases a lot.