This just seems to be standard business case analysis between in house vs buying a product. In general management won’t really care about the technical pros and cons, all they will care about is the financial pros and cons. So your starting analysis should be:
Is it cheaper to produce it in house or is it cheaper to buy a well supported package in order to meet the given technical requirements?
And you need to be brutally honest in your cost analysis. EG my professional time is worth about $1000/day, but I doubt I could produce a product to the quality levels (and more importantly - correctness!!) of Sepasoft before I hit the Sepasoft purchase price. And that’s not including ongoing support and documentation etc.
OTOH in a past life I did build a custom OEE system for a manufacturer. But that was because it had to slot into a combined PLC/SQL server based system which had no possibility of plugging in a separate commercial package. And while the system worked, it was very limited in its functionality.
OTTH (third) this equation changes if you are in the business of building custom software solutions that will be sold to clients (EG you are a competitor to Sepasoft). But from the way your question is phrased I doubt that this is the case.
Been there. Did it in fact, 18 years ago. Oracle + custom C PLC comms + Apache + PHP. Nowhere near what Sepasoft offers, but met their (peculiar) needs. Customer spent over $20k on my work. They were delighted with the result. Until they wanted upgrades ($) and expansions ($$) and custom reports ($$$ !!) and …
Sepasoft was not yet in the picture back then. No commercial solution was available that covered their needs, so they really were choosing between home-grown and a custom contractor. Not the same decision tree, but you can extrapolate the pricing.
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?