The pink fiberglass insulation will probably work, but this type of insulation is very prone to absorbing moisture. A lot of panels today are using polyisocryanate (Styrofoam with the foil facing), which has a pretty high temperature rating and is resistant to moisture.
As far as return line location on the panels, it sounds like you will need to run external manifolds for both your supply and return to accommodate your drain back function.
A tip on collector tilt, you should assess what season of the year you desire the most efficiency from the array. A lower angle will generate highest efficiency in the summer, while an angle closer to 70-80 degrees will produce more heat in the winter. A lot of us solar hot water users keep the collector tilt steep (nearly vertical) to maximize winter heat collection.
My website shows a lot of different recently completed systems; 90% of the material on the site is dedicated to projects and is informational in nature.
www.aluminum-solar-absorbers.com You should find some information of value there.
As the other Tom alluded to, the collectors you found sound like a no brainer to purchase.
Tom Sullivan