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Load rating on tires is based on the gross vehicle weight rating or the weight when fully loaded. GVWR varies a bit year by year. Post LCI seems to cap at 5160 lbs. Pre LCI at 5214. Tire load index is the amount of weight a single tire can carry. Let's use 5250 as our target number. That's 2625/axle assuming a 50/50 distribution or 1312.5/tire. A load rating of 90 (1323 lbs) would be all you needed. However once a car is in motion axle loading is subject to acceleration and braking weight transfer. Let's assume worst case of 60/40. Now loading is .60x5250=3150/axle or 1575/tire. Now a load rating of 97 (1609 lbs) is recommended. The lowest I know of BMW using is 98 (1653 lbs) so that's the minimum load rating I'd use.
TL/DR
A 97 load rating is probably OK, but I'd recommend at least a 98.
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