My primary job is as a project engineer, and I do this daily with suppliers and clients worldwide.2) Fly in early in September/October to be physically present. 

You should always have the attitude that your investors money and them being paid back is THE priority, and getting subs/materials suppliers paid, etc. comes far ahead of management fees!

If you had made a note at 25K and had such an event, you couldn't advance additional amounts, you'd need to make a second mortgage and states may have minimum loan amounts for second mortgages, if that minimum is higher than the funds needed to be advanced, you'd have a real kink and may then need to loan more than was actually needed.The dual signatory checking account may work, but most banks don't accept liability of the two signatures, which means, if the contractor writes a check for materials the supplier is not aware of the bank requirement and if funds are available the bank will clear that check.

After you own the property put together a team that will help you, accountant, lawyer, handyman, plumber, electrician, insurance agent, your family, answering service, discount material suppliers such as appliances, paint, building materials, carpeting wholesaler, etc.I guess you have to start by deciding on a hands on or hands off operation.

Find out where they get materials and supplies and call the supplier for a reference.

In CA, and in my practice, subs cannot go inside my project site without registering their suppliers or another sub-tier, lets go back a bit, you as a homeowner, should NOT allow subs if there is no documentation that your prime is working with the sub, and you should issue the checks on the name of the SUB and PRIME. if you don't want this route, just register the subs and have the prime issue you a conditional waiver then after you pay him, give a penalty clause of $50/day if after 7 days he wont issue a unconditional waiver, have him sign something for every received check the minute you hand it to him.

I guess this is because certain other suppliers have taken a big chunk of the market away?

My GC owns a kitchen business as well and got the cabinets from his supplier.

Go to your local large lumber yard or large supplier and ask the service manager to make recommendations for the type of contractor you need.

I do the same as J Scott - W9 from all contractors, and none from retailers or other suppliers.