Early on, you also need a heavy unit plant to build the major fighting vehicles and an air unit factory. Don't forget things like depots, hangers, and, where appropriate, shipyards and docks. Everything is necessary except in special circumstances. Once you have these facilities working, you are left with deciding which units to build in them. In the early game, scouts are always useful.
Extra surveyors can be good if there is a lot of area to check for vital resources.
Engineers and supply trucks are good. If you get into a fight early on, then bulldozers should be built as soon as possible to take advantage of the debris. Sometimes the debris you pick up off a battlefield can keep a colony alive until the second and third mining stations are built. Which air unit to build is always a good question. Air units are fragile.