This is exciting because I've been thinking about the same thing but more for eco-friendly, modern dwellings for lower income families.

Chris T.While we are not flippers, we do use a similar approach by purchasing tired properties and forcing appreciation: it is just rehab-n-hold as opposed to rehab-n-sell.When we are carrying out a deep retrofit on a newly acquired property, we will put a sign in the yard advertising another "modern, heathy and energy efficient home in the making"  with an indication of when it will be available for rent.We typically have the units rented before the renovation is complete.

With respect, private money is not a commodity item like a book or laptop that you can buy online, and perhaps enjoy the protection of a credit card.

I wouldn't disregard modern heat pumps without real data.One caveat: Installing gas into a home that doesn't have it will significantly drive up your initial cost.  

Changing around floor plans for more modern flow. 

One buyer wanted me to rebuild a chimney because it wasn't up to modern spec.  

Here's an example that is contempo, clean, simple, metal with wood (for continuity to the floor), and it mounts in the same way as your existing balustrade:https://www.hubush.com/modern-stair-railing-ideas/...The hanging fixture examples might be a bit ornate for your architecture.

The house is most likely dated, as I presume he has not modernized it in recent years!

Does anything need to be modernized?

They're all shiny, new, and modern.