Guidelines / Requirements
If this is in any way governmental, there will be some kind of requirements and/or guidelines you need to work by. I don't know where you are but I'm sure this will be the case anywhere. If you havn't been told about these, look for them. Theres a possibility someone isn't doing their job properly and you finding out after spending a big chunk of your time doing things wrong.
There is no need, at all, for any website, web-app, whatever it is, not to be responsive. You say it will only be used on PCs, do you really know that? Who's to say the department doesn't get funding for a bunch of new iPads in 6 months?
Design for Usability
Remember the people using this are working, this is a tool for them to do their job. Try and keep it within what people are used to, if you have access to any of the other systems this department uses, look at those first. Don't try and make it 'pretty', don't try and make it 'fun' and don't try to make it too personal. Make it clean, functional and accessible. That's all you should care about. Ease of use is your number 1 priority.