Mods are hit and miss with this. Problem is SCS is continually patching the game (which is a good thing IMO, because they are quite active on it) but also creates issues with mods. And a lot of them don't play nice with each other. Combine this with dubious means of downloading via having to register with forums it becomes a hassle.
Usually you want to backup your profile or start a fresh one before you do too much.
Take a look at the
Pro mods map pack. Fairly well polished.
There is also TSM which greatly expands the map (which I'm currently using) but will try out Pro mods shortly based on what I've seen. Unfortunately they don't coexist with each other.
Realistic physics plays well with both of them AFAIK.
Combine the above with the fact that this is quite a popular game in many countries and you have language barriers to overcome as well.
I know I don't sound like much help, but I've been searching for the holy grail of a combination of mods that work well together and haven't found it yet.
But the next big patch from SCS will supposedly address the AI drivers of the game. That should be interesting.
One more thing, the steam discussion board for the game isn't half bad and has a lot of useful info.