For just simple 3-5 gallon WCs is it really that crucial? Just sick of going to Aquamart and buying RO every week. I should add my system is very well established and only 30 gallons.
I was planning on adding the DI with just a simple canister from BRS. I think most people prefer the DI to pull as much out of the water as possible, but unless you're going for SPS you'd probably be okay without it.
You want the DI. I have this unit and it runs about 12 TDS post RO. I run it through a BRS DI canister for a 0 TDS post DI. You never know what the RO might miss that will cause you headaches down the road.