Since you are--wisely--taking your time about getting this up and running, I would put a little aside every paycheck until you can get a RO/DI system. I went the same route you did. I'm on a well so no chlorine issues, the mineral content is good, I figured it would be ok despite what the experienced salties here were telling me.
They were right and I was wrong. I got along ok for about six months after setting up with live rock and sand, and working up through turbo snails and then into three or four fish (this is a 29g tall). Then the problems started creeping up: cyanobacteria mostly. Red goop all over everything. I would get the powder treatment stuff, it would clear up for a couple three weeks then it started in again. Le sigh. Finally gave up, tore it down, sold most everything back to the LFS it came from (for pennies on the dollar of course, which serves me right for being an idiot) and went back to fw only.
It can probably be done. Maybe you have better basic tap water, mineral-wise; almost certainly you are more diligent about water changes and maintenance than I am since it would be almost impossible to be worse.
But just sayin', for a long time happy experience with a salt tank find a way to go DO. You will not regret it.