One of the top-end models is the best. Whatever best means. For portraits I would make sure to get a model with an integrated viewfinder, unless you shoot your portraits from a tripod.
If you are looking into adapting more lenses, I would however suggest a camera with a less severe crop factor than what the MFT cameras give you. For only shooting portraits with only the 40mm Summicron, MFT surely makes sense. If there is any further need for the camera, do consider the system(s) more thoroughly. I find the MFT system to be quite alright as long as you stick to the (top) native lenses. It gets less interesting against the competition (such as Sony), once you get into adapting lenses. For some stuff, the crop factor is not that much of an issue. For the wider lenses it surely is.