My assumption is that a lot of renters are looking for more modern style tubs.

There's a positive trend in creating Joint Venture Real Estate deals, with modern communication and business software implementation there's an advantage to having multiple partnering companies.

Yet they're very old fashioned and risk averse, so I'd have to present this well.

All the block was poured full of concrete with a Lentil type block and bolt downs to meet modern wind code.  

To what extent would it be worth renovating to "modernize" a place for a rental?

Fashionable is gray or dark thick(25mm, 15 mm) with good backings. 

To our surprise, during the due diligence period we were allowed to enter the the property and it was in fantastic shape, built in 2008 with modern finishings, no structural issues, and very light cosmetic touchups needed.There was existing month to month tenants paying only $1,100 / month.

It will have received a full remodel inside and out with rustic flare and modern touches.

The hardest part was finding furniture and modernizing the unit, while also giving it nice touches.

Historic timbers can fetch a pretty penny and are very fashionable in some design circles.