Thanks for the kind words! We had a flat roof on our garage at the last place we owned. I didn't install it, but it was very well done with no water issues. They used rubber roofing like this: http://bit.ly/2xY4Zbe and seam kits: http://bit.ly/2xjojT9 The downside is that it is black and gets pretty hot on a sunny day, that was never a problem for us, but I suppose it could be in warmer climates.
As far as insulation, we just re-did our house roof last month and had it sprayfoamed for about 89 cents a sqft/1" thick. Not sure if that is in your budget, but it is by far the most efficient type of insulation I have found. We did about 4" of sprayfoam and now I am following up with some fiberglass insulation for a little additional insulation and sound dampening.
Good luck on the house project!!
Thanks for your advise. I really appreciate this.