You can set up which states newly-created units and structures have by default in the options menu. For example, I always set all of my factories on repeat by default, so that they start out ready to continuously build whatever I want them to build frequently, thus saving me the need to have to babysit them whenever they finish building whatever they need to build.
If I want to make those factories, which have been set on repeat, build just a handful of a specific unit without repeating them, I hold down the Alt key as I pick which units to build, so that not only are those units NOT built repeatedly, but they are inserted right behind what's currently being built (which is good if you want some units made ASAP).
As for queuing up orders for your units, including construction orders, holding Shift should do the job. If you need something done right away whilst preserving your queue of orders (such as getting something built, or taking out a certain enemy, right away), holding Space should help. Finally, you can add tasks in-between existing orders by holding both Shift and Space.