I have built many systems with stock management components.. but would require a lot more detail to see if any would fit the spec. If I were to build such a system, I would probably start from scratch and copy/paste the pre-built elements into a new system.
Obviously, such a system would require hosting from FileMaker Server... then you have the additional option of using Instant Web Publishing if you use FileMaker Server Advanced. That is the starting point for 'industrial strength" along with the hardware and network capacities you choose.
Then... at the client level... what functionality to you want each group to have will determine system security requirements and potentially interfaces. Do you want to have every user accessing the database via FileMaker Pro or are there cases where a web-browser would be more suited to the user.... I would suggest a combination of these might be required.
Learning resources online from FileMaker are very good (just look in the Help menu) but there will definitely be a learning curve and sometimes the best way is to work is to have someone to call on to plan-with and who can answer the questions about issues as they arise. The other thing you need to consider is whether it would be easier and cheaper in the long-run to get someone else to do it for you.
Just like many others on this list... I do both... remotely for all my clients. I would suggest you contact a developer of you choice in a private message to make such an arrangement.