Zeckhausen sells good products, I wouldn't hesitate to buy from him, and would have confidence that they will install and fit right, even if its not exactly the same as stock. As long as the fitment is good I see no negative effects of installing braided stainless lines, if they are cheaper than stock that is a plus for me, in my VW days stainless lines were usually a price premium.
ECS also advertises an exact fit Stainless line kit for all 4 corners for $95 if you're interested:
BMW E53 X5 M54 3.0L > Braking > Lines > ES#2609185 Exact-Fit Stainless Steel Brake Lines - Kit - 002569ECSKT
I would go ahead and replace lines at all 4 corners if that is the route you were going, give it a nice brake/clutch flush too while you're at it (see I'm always adding projects for you!)