I'm putting some 17x7 et37 wheels on my 06 mcs. Will 205/45s work without major rubbing issues? The car is on the stock suspension.

205/45 will work fine. those wheel specs at stock height aren't really restrictive in terms of rubbing.

205/45-17 will not rub.

215/45-17 will not rub with stock suspension.

215/40-17 would be smaller tire diameter and give more wheel gap.

You will start to get into trouble if you drop your suspension by 1" especially in the rear. That is due to the et 37mm aggressive offset for the rear.

If you use coilovers you can adjust ride height to fit any tire.