Now I'm leaning toward the UM2 for this build.
I would like to mount the steppers on the outside of the frame for proper cooling and longevity of the motors. I reffered to this as "reversing the steppers". Sorry for the vague phrasing.
Cutting holes and mounting the motors on the outside of the UM2 seems fairly easy in principle but have only seen the UMO & UMO+ with this modification in my limited research.
I have not customized or built firmware before but i imagine it will be very similar to @rajilpahuja since I would like to use E3D V6 hotend 1.75mm, bondtech BMG, and AC Silicone Bed heater. One of the differences I see between @rajilpahuja would be mounting the stepper motors on the outside which can be done at anytime and is not crucial to printing as seen by all of the work he has done.