Im sorry but I would strongly disagree with not needing to get a RO/DI right off the bat if you have means to hook one up at home Then that unit should be budgeted in to your set up cost, For the plumbing part of it , That's a 10 min job to hook and plumb one up. Heck you can hook one up to your water spigot and drain it in your yard or on you plants. You can get a nice RO/DI unit for about 150 to 200 bucks. Trust me it is a pain in the *** to have to worry about going to the store everytime you want to do a water change and pray to god the store you get it from changes out there filters like they should. Besides it never hurys to have fresh water on hand in case of a emergence.
Just my 2 cents
I live in an apt. I always keep spare water on hand, but plumbing it in, no matter how easy, isn't an option. Also, Not all hobbyists will stay with it, and if you are on a tight budget, it's not strictly necessary. Just my two cents.