Not my cup of tea and not recommended unless you plan to be there and keep a personal eye on it for a long time. 3.

are there warehouses ... direct window suppliers, direct flooring/doors/fixtures without home depot or someone like a Pella or Overhead Garage Door?

As a Post Script to the lease signature I state:The drain and sewer lines are all operational and that is my responsibility to you.However, if a fixture becomes blocked, that is your responsibility, not mine and you need to seek profession service to clear it. 

The kitchens and bathrooms have modern fixtures, appliances and decor.

My tenants have paid to upgrade all the apartments and buildings that I have ever purchased - new windows (every single one of my units has them), new floors, doors, fixtures, counter tops, appliances, paint, etc., etc.

The sediment filter catches all the big particles and keeps them out if the softener; the softener is vital for protecting your plumbing, fixtures, and water heater; the RO system is important if your tenants plan to drink the water (softened water is not all that tasty).

This usually includes new roofs, gutters, flooring, cabinets, bathroom fixtures, counters, etc.

Still, I envision me starting very small to see if this is my cup of tea.

They fit directly into the drain pipe on cup over the toilet waste flange.

By the way, for any onlookers, this is how I like to think of price per sq ft and how I tend to explain it (yes, with Starbucks cups): Best wishes.