I think many agree that GTA V cars are great in any DARKRP, although it is a matter of taste, I personally like them. The addon: GTA V Drivable Vehicles for Garry's Mod
It is not an addon that is necessary for DARKRP but I consider it an important one when it comes to mounting custom content to your server, it is easy to use, you add it to your collection and voila, it does all the work for you. The addon: Ultimate Workshop Downloader
If you want to change the scene that is not always the same rp_downtown map, I recommend this one, it is 1.5 times bigger than the downton and offers more variety and different streets, personally if I had a server, I would use this map. The Map: rp_eastcoast_v4b