I would do the sales(APSS) and the design (APDS) as well , true there is no requirement but it will give you a good idea of the building of the systems and prepare you well for your ACIS , i have just passed my ACIS (ACibIS - Avaya Aura® Communication Manager and CM Messaging – Embedded (R6.x) (ACIS - 6002) and am working towards ACSS , but you do need to select a path and stick to it i find the best way is start with the sales , then the design and then the installation as this way by the time you get to the installation , you are familiar with all aspects of the system capabilities / build and have a good feel for what the format regarding questions is like.
There is an awful lot of reading up if you take all three proccess on , but it really is worth it in the end.
APSS (SME)
ACSS (SME)