Ok, you deffinetly need a ph and a ppm meter, but under your circumstances (having RO) you can do without for a min. Your RO water should come out 6.0-6.5. The nutes your using have buffering solutions in them and will drop it depending on how much you put in. You gonna have to change your solution out a lot more than you would normally to keep control but You should be cool until you can afford one. But get a ph meter ASAP.
GL with the grow..I have a very similar setup so If you need help just ask. Later