The short version is - Seek professional help.
The longer version:
You appear to have some vague knowledge of what you want to do but certainly don't have the specifics. What you have here is an entire Corporate GIS project and you should go through the Project Management process for it.
What are your requirements? What do you need to connect to what? Who will need access to the information? What will be readable? Can anyone write to it or only specific users? How often will data be updated? Do you need certain tools or functions? And on and on.
Because much of this is specific to the GIS domain, you're likely going to need to ask someone with GIS knowledge; if you don't have anyone on staff then get a consultant. They'll tell you what to ask and help you find out the answers.
Once you have all of those and have your detailed list of requirements, then you can start asking specific questions about which GIS system will meet your clear needs. Its then a procurement exercise to buy it externally or you could build it internally if you have the capacity (maybe built up over the "what are the requirements" stage).
Edit: You may also find these questions helpful, both to specifically deal with issues raised in your question, and also demonstrate the related questions that elicited good answers.
Managing large amounts of geospatial data?
How do you manage existing GIS data and allow for integration with an Asset management system?
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Best example of a GIS project gone wrong
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