What I've found in my C, if you have an adult in the backseat that's anywhere near MIL-Spec isn't an issue. If they have long legs, they can set slightly sideways. In adding a 4th passenger, I have to start leaving fuel behind which digs into most of my intended missions. So for me, it's two person long distance trips, three for shorter (2-3 hour) trips and only local flights with 4 on board. The only time I really wish I had a mid-body plane is when I'm packing for a trip. That additional 5" of baggage would really come in handy. I find I run out of room before hitting the baggage weight.
As far as avionics go, these planes are for traveling so I personally would not consider purchasing any Mooney no matter what model unless it has an operational auto-pilot (preferably w/Alt hold) and WAAS GPS. These two are very expensive and it's much cheaper if you find a plane that already has them. other items like engine monitors, etc can be added later.