How much const management experience do you have? Logistics and organization is usually the key factor in bringing a project in on budget. Another is dealing with subs, which more often then not will fuck up the LnO
Another thing with doing 40% of the work yourself, do you have that kind of time outside your regular job? My uncle built his country house after he retired (at 57) took him about 2 months, appraised house alone 300k, built for a total of 65k. My Dad built his while he was still working a 40-50 hr/week, took a year and a half, valued out about the same, but cost 30k more to build.
The difference is that while my uncle was building, if it was time for plumbing, the whole house was ready for it. My dad, would have "buddies" do it around their regular work schedule, this part here, this part there.