Rehabbing an Apartment complex!!!

[Summer Azul]

One of my main goals in Real Estate is to acquire an apartment complex of 20-50 units. I have found several great deal on building that are in need of renovations. I am ok with the rehab aspect as well as the cost and time frame etc. My biggest concern is the rental income amount. How do I determine what the rent should be for the units? Am I able to charge more based on the upgrades that I install during the rehab? Lastly, I often times see apartments advertised for different amounts but they are within the same building... Is there a reason for the price difference?