You are probably fine to use distilled water if you are using extract and will not need additions - the minerals in the wort before it was made into extract will still be in there.
If doing all grain, yes, add minerals depending on how you want your water.
If you are doing a 5 gallon batch, 5 gallons of distilled water will not be even close to enough. Even if you are doing extract you will need more water to account for boil-off. Not to mention other losses associated with all-grain (and incidental water losses with extract or grain).
So buy more than 5 gallons.