D-pad is currently not assignable directly, however, 3rd party programs like XPadder or Joy2Key work well. You can even use them to bind flashing lights and ABS and TC to your T300, I sure have on mine
PDash is a good mod dashboard, though I recommend you get an old smartphone to use as a dashboard via FastDash or some other app. Immersion goes up 75%.
Real Head Motion is brilliant, especially if you use a single screen setup. Not essential, some don't even like it but I can't play without it anymore.
RSR live timing is a nice little app, they have a new build and leaderboard reset coming up soon. Also PTracker is just great on supported servers.
Plenty of good mod tracks and cars, though the game is fine without them as well, so I won't go into all that.
Enjoy your stay, AC can be one hell of a racing game
EDIT: Almost forgot, the natural mod PP-filter is bloody brilliant. And Fonsecker sounds, they add a lot to the experience. (Z4 and Ruf CTR are the only bad ones in his pack IMO)