If most of the neighborhoods have more modern kitchen's though, it might cause a drop in rent value.

I needed a more modern, easy to understand interface. 

Just my 2 cents.BONUS TIP:If you are at a yard sale or on Craigslist and see old tools made here in the states they will probably outlast some of the modern stuff you can buy at a big box store.

However, in order to charge a premium price your home has to be of premium quality, so I would recommend modernizing the home with decent flooring, appliances, fresh paint, laminate and tile flooring (no carpet), etc.

If it was a modern home utilizing PVC they shouldn't fail but I have had the Johnny bolts break through the PVC boosy flange. 

If they were installed well and were a bit more modern we would probably be keeping them, but as it is they are an eyesore to the tenants, very difficult to get clean at turnover, and are not sealed up very well.

I'd put a nice carpet runner up the treads, it will protect the treads and should be fairly cheap; pick a good wearing but modern pattern/color for the carpet.

This will assume a lot and won't fit every case, but can serve as a benchmark. 

., super fancy granite counter tops) but "solid" (mid-level in price/quality) that give the ADU a modern look and feel and a foundation of materials that would last a fairly long time without being refurbished.She is getting quotes with a fairly wide range, from $140K all the way up to $190K.Thanks in advance!

They need to meet or exceed the income benchmark you've established in order to potentially qualify as your tenants.