New system help

I'm installing a new rad floor staple up system in my son's 1500 sq ft single story house. We will be using 1/2 pex with 2 lines in each joist cavity.  I have used a watts mixing valve with success in other systems I've installed,but I'm wondering if there may be a more efficient piping scheme.  He is on a minimal budget so mixing blocks are out of the question.  We will have 3 zones, 2BR zones and a LR/ KIt zone.  We have 007 pumps (used but in good cond) available to use and are hooking to a Weil Mclaine CG-4. . The house was built in the 40's with little insulation.  As he can afford to tear out the plaster he insulates the outside walls, and there is 9" of glass n the ceiling. I'm open to thoughts opinions and drawings