This is the main AC thread, there's tons of info on everything there
http://forums.overclockers.co.uk/showthread.php?t=18355863&page=115
1. Mods:
Cars
http://assettocorsa.club/mods/auto/
Tracks
http://assettocorsa.club/mods/tracks/
The best mod cars are Lamborghini Miura, some F1 cars like the McLaren 1998, Honda NSX, Mazda 787. I also like the Countach and Nissan 180sx
Tracks - I only have the modded Top Gear which I love.
Settings for DFGT - there should be some in the AC thread above.
EDIT: one last tip - almost forgot!
- turn ON autoblip to help with downshifts on manual cars (since you don't have clutch pedal and cant do heel-toe), but turn OFF traction control for maximum enjoyment