I think a balloon would be great for assisting a craft achieve altitude without those expensive booster rockets, but on the other hand if a ship where to be loaded down for a three year plus mission to Mars then that would have to be one heck of a big balloon if it is even possible. I think the building a skyscraper to jut into outer space and act as a loading dock for the crew and supplies of such ships would be better because the building could also have other uses. I think the negative side of such a building would be the huge cost of maintenance and the risk of terrorist attack.