If you're planning on going reef later on down the road, I'd recommend getting RODI
membranes filter approximately 95%-98% of contaminants from the water, and DI
resin removes the remainder. The problem with this is you don't know what
is in that 2% to 5% that makes it through the membrane, and many of those nasties can slowly accumulate in the pores of your live rock, eventually creating a high enough concentration that can be detrimental to your system.
To your second question, yes, you can pre-mix saltwater & store it, but it will need to be brought up to temp prior to use. If you're mixing small quantities, you're better off just mixing it the night before with a small heater & powerhead so it's ready the following day.