With an eye on replacing said information systems, I've had a look at the open source medical records / hospital management systems available. When I looked at the details these systems are often not great replacements. So you're replacing aging, poorly written information systems with aging / non user friendly / difficult to customise information systems.
I would like to suggest some things for you guys:
1. Instead of creating one large hospital management system from scratch, how about smaller systems that can be linked together? eg. patient records system / laboratory system / pharmacy dispensing system / billing system / etc. The systems I mention here have fairly minimal dependencies between each other. This gives you the time to create a best of breed system before moving onto other stuff. It also allows hospitals to be able to use your stuff without ripping out everything they already have!
2. Think about how a hospital would customise your system. New fields, forms, reports, workflows, logic, etc. And how these customisations would survive an upgrade of the core system.
Anyway, I hope you have success with the project and I wish you luck. I'll definitely be keeping an eye on it!