The house was professionally painted with a high-quality antique white paint before these tenants moved in.

My preference is older, antique type furniture just because of construction.

The furniture which will be conveyed with the property is a combination of extremely frou frou antiques in combination with a mix match of whatever was available to be found in local junk stores for a reasonable price when the sellers bought the place 40 years ago.

We had to replace our old hinges and used a site called House of Antique hardware to get ones to match  You can also check your local habitat store, but we found them to be horribly expensive there.

Salvage the antique pine and use that as flooring over the new subfloor.

It's similar to haggling at a flea market over the price of an antique except that your first offer is in writing instead of verbal.

Does anyone invest in and manage homes that have old or antiquated plumbing systems? 

If only they would legalize marijuana you would be sitting pretty :) But in the meantime, the only thing I have seen successful is like an Antique Mall/fleamarket where you rent the space out to vendors. 

You can also find antique looking grates for cheap on Amazon to put over the holes assuming they are standard sizes.

.• Their third party provider of processing (Digitsz) is antiquated and super clunky.