I am hoping to get some input as to whether it’s worth the effort to change out the tub for a shower, and suggestions on what sort of flooring would be easiest/most durable/most economic to put in.

Shipping containers are inexpensive and durable buildings for residential, commercial and even industrial use.

Then, I would pay considerable attention to securing the property, install some seriously durable flooring and a fire sprinkler system.

Durability, at least 5 years.

It has a higher PSI for durability. 

I loved the HostGPO recommendation, but personally haven't been able to wait as long as much of the furniture takes so have sourced from Cort, Amazon, Wayfair and Walmart with great results--I've built a tried a true list of reasonable, attractive, durable and easy to assemble items that I use again and again.

Tile is durable but cold up here. 

Floors are typically LVP rather than laminate floors as they are more durable, and hold up better in rentals.  

I want it to be very durable and trendy but not too expensive.

The key is, define the scope for them, don't let them do the scope because they are going to install the cheapest item, that could be a difference in durability, and wouldn't really be a fair bid.