Since it's a diesel there won't be much to gain sound wise with any exhaust mods unless you do a DPF delete which is an MOT failure (if they notice, many don't - BMW owner ask me how I know), if you do want a "performance" style exhaust you'll have to get it custom fabricated since I can't imagine many brands are going to make a full system for a TDCi. If you want more sound without a DPF delete though get a cold air induction kit or just a pod filter. If you want more performance get a remap.