:duh Looking to fit some benz rims to a bay combi. I've been told the et should be between 37 & 51. Is this correct?

sounds about right.
the Rims I had on my V6 Bay came off a merc.
Don't go too nuts with rear width though, some merc rims are 9" wide.
Stick around 7-7.5" and you should be ok.