99.9% of mods that work in Race07 and GTR Evo or STCC will work in Race On, and if they don't it's quite easy to fix them so that they do.
And if you look around the web and here there are a number of very good mods available, the Mustang mod as David has already mentioned, the Ford GT GT3 by someone whose name I forget (
), the GTL-Evo mod by Mojo (although that could be a bit hard to obtain these days for reasons best not mentioned here), the GTR2-Evo mod by Petrs and co., then there's the Porsche Carrera Cup for Evo and the Audi R8 Widebody by ZeZeZoom, and there's the Porsche 935 by Majoteam and that person whose name who I've forgotten. (
There's also a couple of open wheel mods for Evo as well.
Whilst it's true to say that Race07/Evo/STCC/Race On doesn't have the number of mods that are available for rFactor, GTL or GTR2 it still has some very good mods available, if you look around and read.
The reason behind it not having so many mods I believe is that the guys who mod for the SimBin/ISI titles just don't really like the Race07 platform due to the nature of the way SimBin locked it down and removed a whole load of options.
But I'm sure there are other reasons as well.
But yeah have a deeper look around, you'll find some gold.