Let me start by saying that I've never used a Mamiya 7.
That out of the way, I have extensively used a Mamiya TLR with 80/2.8 and 135/4.5 lenses, as well as a Fuji GS 645s with a 60/4 lens.
I sort of cringe when I hear people complain that medium format lenses are "slow" at F/4 or so. Sure, you aren't likely to shoot in a dim bar and get great results, but as far a DOF is concerned these lenses are directly comparable to 35mm equivalent ones at least 2 stops faster. DOF with a 80 to 150 lens in any format is going to be REALLY thin- like more lens speed wouldn't be very usable.
As far as portraits are concerned, the Mamiya's 80mm lens is just about comparable to to a 35/2 lens on a 35mm camera, and the 150 is about like a 75/2. Only with much smoother OOF, tonality and sharpness. The 1.8M near focus limit is probably limiting though. As said above, you should be able to do a 3/4 length portrait; tighter than that and you'll have to crop.
Would you be better off with a different piece of gear? Depends how much you value portability, ability to shoot without a tripod, versatility and cost.
Is there a way you could rent before buying? That would go much further toward answering your questions than anything.