Keep them evenly moist while they are growing rapidly early in the season so they develop an extensive root system. As summer progresses, they become more and more drought-tolerant. Zinnias grow in almost any soil as long as it is well-drained. Their drought-tolerance makes them a good choice for sandy soils. Zinnias aren't heavy feeders.
All older zinnias are suitable for cutting.
In fact, one variety is called 'Cut and Come Again'. The old standard for a large flower with a long stem is called 'State Fair' and there are lots of newer improved varieties. Check the package or the tag for plant height. If it's more than 18 inches, it will make a good cut flower. Ideally the flowers should be cut just as they begin to open and the stems should be immersed immediately in lukewarm water.