Get a tds metre and check your water tds mate.
Anything under say 80 use d.i only, the amount you state you'll use it that'll be fine.
Anything over, you're better off fitting a 4040 r.o, especially if you just want to attach hose and produce straight into your tank.
As regards to system, personally mate, i'd fit a DIY system together yourself (see if a local guy will help you or show you theres.)
Its alot easier than you may think, and you'll know your system in and out.