Hey Keith,
Welcome to the site. Everyone uses a different type of holding tank, its simply a matter of budget, size, color, shape, and what your tank needs. Some of the bigger tanks need enough to accomplish enormous sized water changes and huge amounts of daily top off so they use vats. Others (myself included) only use 5 gallon buckets, or a 32 gallon new plastic trash can. Its completely dependent on your situation.
As for a float system for your tank, you have a couple options. You can use whats called an ATO (Auto Top-Off) which more specifically, is electric. It uses 1 or 2 (preferably 2) float switches to activate a pump that pumps fresh RO/DI water (no salt) into your tank to a set level and then shuts itself back off. There's a number of different brands to look at, some being relatively cheap, others are up there in price.
The other way, which I employ, uses gravity. I have a 5G container that sits higher than my sump, and has 1/4" plastic line running down to a float valve (no electrics) in my sump. The valve is set to a certain level and works awesome and was very inexpensive.
To top off the holding tank, you'll want to do it manually. Reason being, is that the float system that is used with an RO/DI can fail. So, you have the potential for a big flood. Virtually unlimited water supply from your wall, a valve that can fail, and you have a recipe for disaster. Alot of this hobby contains Murphy's Law, this being one of them.
Pics to include, none are mine:
