auto top-off

I'm interested in this as well and my tank is 30g. I'm looking for something complete. Unless a DIY is super easy.
If you have the cash, The Tunze Osmolator is hard to beat.

Otherwise, check out

A two Float Switch unit with a 12v power adapter upgrade (I highly recommend this, decreases the odds of fried float switches) will run you about $70 shipped.
WOuldn't tie a ro/di filter to the topoff unit: a flood is a measured flood unless it's connected to the city water supply.

A concealed tub filled with ro/di is the best supplier. I used a 7 g. old salt bucket for mine [54g tank] until I moved and put everything in the basement. Now I use a 32g Brute trashcan and only have to fill it once every few weeks.

I use the topoff, which is simple, pretty intuitive, and thus far snailproof in my refugium.
I wanted someone else to mention the Osmolator from Tunze before I did. I had one on my old tank and it rocked. Totally reliable. Talk to Roger in the Tunze forum for more information if you're interested.
There's a simple gravity fed design that I use that's cheap and fool proof. I wrote up a description a while back... let me see if I can find it....

You need a glass water bottle... plastic will not work, it's not rigid. You'll need three lengths of rigid air hose, one the length of the depth of the bottle, the other two about two inches long. You'll need two rubber stoppers, one for the bottle and one for the PVC straw, and you'll need a piece of 1/2 inch PVC to act as a straw. Lastly, you'll need two lengths of flexable air hose.

Drill two holes through the bottle's cork and force the two air tubes through them, one reaching the bottom of the jar, the other just reaching the other side of the cork. The one reaching the bottom will be hooked up to a piece of air tubing for siphoning water out of the bottle. The other will be hooked up to the PVC straw. The PVC needs to be corked at one end, and long enough so that the top of the PVC straw is as high as the top of the bottle. Air holes are drilled in the PVC at the level you want the water to be in your return area of your sump.

Start a siphon, and water will be pulled from the bottle into your sump. As it does, if there is water above the air holes, water will be sucked up the pvc straw until it's the same level as water in the bottle. Once air is allowed into the straw, the siphon will automatically resume to replace the water.

There's a picture of the set up in my gallery.