It's a science but not a perfect one.

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.

It's definitely an inexact science, but those kinds of things help.

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

Does anything need to be modernized?

Of course an upstairs will be warmer than a downstairs; heat RISES (these morons must have slept through basic science in high school). 

They're all shiny, new, and modern.  

The SFR you mention sounds like the way to go for this reason: It's modern construction and it's attractive to a larger pool of buyers.

I bought all mine for under 50 a ft...They are newer modern layout and were realativly turn key So I have not pursed building beyond getting  my budgets.

It's a not an exact science but a great rule of thumb to quickly determine feasiblity of a rental.