I had the house painted for $175 in materials + $200 for a friend to help.

5 years is a long term plan and a lot can change in that time, you could have a second child, a move, or even a significant job change or promotion. 

I have already started on the Podcast and the reading material.

We netted $30k, which I split with a partner who did the work and put up the materials.

What’s the best way to handle the upfront money when the contractor says he needs $$ to buy materials?

I would love to go into details on how to estimate costs of putting work into it (materials, getting to know contractors, etc) to make an informed decision when purchasing a property up front.

Then it produced a materials list of their windows, doors etc etc that they already had & pricing.

I've established a charge account at Lowe's and i'm allowed to add contractors to my account for charging materials. 

(ps- my partner is a GC and his employees would be doing the labor- so the material cost is the major deciding factor here).

whatever you stance on that, it has created a situation where we (americans) look down on factory built homes.Without the artificial input I believe we would have giant , efficient, factories in every city pumping out modular homes, that are green, affordable and accepted as "normal" and probably cost as much as a nice new truck.If you have ever built a new home, the insanity of "site built" becomes apparent as the waste of materials and man hours can be staggering.