The F35 Joint Strike Fighter is a 1 TRILLION dollar defense boondoggle. The idea was that the US Air Force, Navy, and Marines would all use the same fighter jet. That sounds great, except each branch of the armed forces has different needs. By the time those needs were sorted out they ended up with three models that share only 20% of their components (essentially 3 different jets).
The fighter project tried to be all things to all people. Instead, it ended up not making anyone happy. Don't let this happen to your IT project!
Your goal shouldn't be to get all your departments on one system. Your goal should be to get your departments the functionality they NEED. If all those NEEDS align with each other and make a single system feasible, great! If not, focus on how how you might be able to tie a few systems together either by technology (APIs, CSVs) or good ole' humans!