I am thinking if you do mostly lap and crib size quilts is there really a need for the bigger harp space? I easily quilted up to 60" quilts in my DSM and some bigger ones when I was just doing grid quilting with a walking foot. I have a Bernina with only about 8 or 9" in the harp.
Anything bigger is what I had trouble wrestling with and I mostly make queen size quilts. Just something for consideration before making the investment that may not be necessary.
Edited to add they make plexiglass work tables that fit around DSM that increases your flat space.