I take it you are building this from a supplied Kit or a supplied set of drawings Rik, most people don't buy electronics until all the design has been done & generally quite a bit of the build as things seem to have a habit of changing, Plan A becomes Plan B etc etc as improvements are suggested or the overal size of the machine changes.
If your building from a kit then it won't matter but if your designing yourself might be best to hold off on buying things like motors just yet.