We did find half-a-dozen of the pre-change packages at a local supplier and have put them aside for use in upcoming renovations.When it comes to replacement shower heads we have used the Delta 1.5gpm H2O Kinetic heads (here and here) and find them to work well (use them in student rentals).  

Much may depend on your market, supplier, and delivery etc...I run into whirlpool products and use them but not the brands you may think.

My second comment: The biggest challenges in this concept/project consists of a minimum of: 1) finding a supplier in the Prefabricated Home Manufacturing industry, 2) establishing financing in this space, 3) meeting development and building standards from your local jurisdiction authority.

@Wesley Merville There are several paid probate lead suppliers.   

{Note: we have an environmental condition in our Agreement of Purchase and Sale (APS)}.confirm if an engineering assessment/study (something beyond the normal building inspection) is required;  order if necessary; review results before signing off on inspection condition in APS.obtain copies of any zoning confirmation letters or variances applicable to the property (and its current use);obtain consent letter from Vendor to pull property histories for workorders, citations, zoning variance applications, planning submissions, & building permit history from City/Municipal planning and inspections department;(if applicable) obtain verification from fire marshal that property is up-to-date on compliance and inspections;obtain inspection history and operational certificate for any lifts/elevators on the property;in addition to estoppels for tenants we also review all supplier/service contracts and, if the contract is informal {aka handshake}, we execute an estoppel with the supplier/provider.   

Its an 8 week project for $40K so I am leaning towards $10K check to start as a sign of good faith, and then $10K every 2 weeks with a credit card.I will also pay for materials directly to the supplier.

However, you may find it easier to go through a supplier. 

It will also allow you to buy construction materials in bulk giving you some negotiating power with suppliers. 

Layout, material selection, and supplier leads?

Why not send your layout to two different cabinet suppliers(that come recommended from other flippers), and then you can usually tell where your numbers should be.