My medium-term goal is to own and rent 10-15 properties in the next 5 years with this method.

For quick and dirty method, apply the 50% rule for expenses.If this is a distressed asset, give them a cap rate before and after stabilization.

Another alternative for new construction homes would simply be the "advanced framing method".

House hacking, seller financing, subject-to, flipping, wholesaling are just a few methods that come to mind.

Plumbing, electricity, heating and air - all of that stuff needs using or it give problems from sitting idle too long.Your insurance needs to be specific for vacation rentals. 

You will have two escrows, two closing, and two sets of closing costs in this method and you will also need to bring funds to the table to close the transaction between you and seller.More importantly, assignments as a means to wholesale are typically executed illegally as they violate CA DRE laws for brokering without a license so I strongly suggest that you look into the legalities of your wholesale structure before you proceed. 

@Jim K.I’m a big fan of the Socratic method.

1940s house build on a slab - a good amount of issues that I had to fix with plumbing with tenants along the way that were a pretty big pain being on a slab Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

Not sure if that's how plumbers charge for their services, I know nothing about plumbing.

How much more does the pre-plumbed option cost?