I use a 4 stage RODI unit from BRS with the sink faucet attachment. I picked up two 20 gal rubbermaid trash cans with lids (white for RODI, blue for saltwater) and dolly/caster bottoms so I can roll them around. I have a 43 gallon reef so I generally just need about 15 gallons each of saltwater(waterchanges) and RODI(ATO and 2 part dose mix). I hook up my RODI unit to the sink and I put the buckets into my bath tub and let them fill over the course of a few hours. I set a timer and walk away. My unit does 3 gallons a hour. Putting them in the bathtub while filling is just an overflow precaution.
I like this set up because its not permanent and has an over flow safety. Ive seen awesome set ups with apex controllers that hard plumbs your RODI straight to the tanks ATO and mixing containers with float switches. It is completely automated except for adding the salt. Although there are precautions with this set up because you're plumbing an essentially unlimited water source into your tank. But I think this can be a decently reliable system as long as you have programming in place to kill your RODI line if it gets stuck open by monitoring a major salinity drop on the apex probe.
I would say the 75 gallon you're thinking of getting is right around the size I myself would consider hard plumbing a RODI unit with mixing cans and fill pumps to the aquarium. How much you automate it is up to you. But it is without question cheaper to have your own mixing set up vs buying from LFS.
