So, how does this work then?
I can't help but think that most of the variables are known: What data to store, how to enter it, how to access it, who needs and who may have access, how to handle retention, dropping of records, backups, redundancy, availability, that sort of thing. So setup a pilot containing a few fat filers and a database front, roll out endpoints, hardware, client software, or both, to the various agencies, do it incrementally, and have the thing gradually take over the existing system with most data entry being done by the users (which would be the steady state anyway), and only after it's been deemed stable enough (do stress tests), decomission the old system. That's how certain police forces ended up doing dispatch supported by homegrown MSX software(!) though that ended up as unsupportable as the bbc micro in the end.
How much do you need for that? A couple good sysadmins, an interface writer or two, a documentation writer, a release engineer/toolsmith, one PM, and a couple field teams, say two to three people each, plus assorted hardware. Cost? Uh, a couple million quid and fiat from on high to get started, and scale as needed. I think you can do it for well under 100 million and inside of a decade, first try's the charm.
What am I missing here? Alright, this doesn't work if you're too big to start with, of course.