The FM3a is a great camera and would make a great second body. In fact, it would probably be your main body.
However, since you already own an FM2n and don't really need your other camera to have a fully mechanical shutter, why not consider a nice used FE2? They are very inexpensive and do everything the FM3a does, except work without a battery. (Well, they work, but only at 1/250.)