I believe E90's use a steel subframe, hence more prone to rust, where as I think E89/E92's use a different material.The M3 options is off the table for now. I do need to properly inspect the existing subframe, if the rust is anything deeper than surface rust then I'll consider replacing it.
I found a local garage that will reseal the diff. So I'll buy the seals and fluid then drop it over to them.
The rust issue is a bit confusing really. The front of the vehicle looks good, just some rust around the power steering lines. The rear is a mess.
My E89 has the same mileage and has been used for similar journeys and is spotless. Just some slight surface rust on the subframe. Maybe the first owner paid for some kind of rust proofing spray, or BMW just made the rear end out of better quality parts.