You will still spend a considerable amount of time developing even with the OEE module. It might even be from scratch for certain parts as the stock Sepasoft components aren't very customizable and might not align well with your plant operational needs. The OEE module will collect a lot of data, but you will still find yourself working out how you want to retrieve that data, how you want to transform it into meaningful information be it dashboards, reports, emails...etc. There is a considerable learning curve that you need to overcome to really get what you want out of it and the module isn't without it's flaws.
As for justification that should be easy. If the OEE system (any OEE system) can help identify causes of downtime, performance inefficiencies and inconsistencies in the manufacturing process and as a result actions are taken to reduce downtime by 1%, improve efficiency by 1%, improve quality by 1% across all of your production lines. How much $$ would you save in 1 year?