Ive had 3 E90's now - 2 x 335D M Sports, and a 320ED. The best mod is putting standard tyres on. The Ed comes on standard tyres and lowered suspension, and got 5*s in the EVO test. The RFTs crucify the ride. Other than that, when correctly shod in the right tyres, they are a great drive. On my tourer I put lowering springs on it, looked great but the ride was a little hard at the rear, however Ive driven a saloon with the same springs and the ride was great, better than factory.
The 300i is considered the most pure of the petrol models, due to the lack of turbo chargers, and makes a great noise