How much to upgrade on a rental property

[Michael St. Marie]

Hey guys - My wife and I live in Columbia in South Carolina - she has being in grad school for a couple years but we are planning to move back upstate early next year. When we moved we rented our house out and self managed, it has been successfully rented out for the entire time we have been away (by successful, I mean that it has not been vacant and has cash flowed around 300/month)

We are planning to move into the house for 18-24 months before moving into something else and keeping this house as a rental. 

My question is: assuming, we are going to do some upgrades to the house before moving into it, what rules should we follow about how much to put into a rental property? How do you gauge future rental value when deciding what to upgrade? 

Just a little bit more background, this house has been a rental for many years, we bought it from the previous landlord - It has not been well taken care of nor has it been updated over the years. That said, it is in a gentrifying part of town, and based on the nearby commercial and municipal upgrades is in the future path-of-progress. 

Thank you so much for any advice you can offer!