Should I engage an architect, an architectural technician or an architectural designer on my 2-storey extension project?

An architectural designer or technician is more than capable of providing design drawings and technical detail to complete the project you have described. They will also have sufficient planning knowledge to maximise the chances of your project being approved. If you are looking for a design that includes material and texture options, an architect may be a better option; be warned, such enhanced design skills invariably come at a price.