Di filters are the final stage of filtration. The Di resin removes left over by-products, phosphates and other trace elements that snuck their way past the other filters. If your going with a ro unit anyway get a least one DI filter as the final stage.
No I have not had a problem keeping my levels in check.
A quality salt is going to do that anyway. If your doing your water changes on time> or when levels start to get off balance You don't have to worry so much about alk, ph ca.
Your levels will peak up once your water change is done.
I run a ro/di filter system with two DI filters and my water comes out 0 tds and my tape going in read 289-300 tds.