We know the market and how to handle vendors.
It does not exist a standard e-voting system that can be used without modification. All countries have different legal framework, and democratic traditions. This must be implemented in the system.
Don't try to make a complete specification. Involve the vendors before you decide what to by. We had great success using the EU procurement procedure "competitive dialogue" Had we approached it differently the end result for Norway would not have been as good .
To be a successful customer you must have sufficient knowledge on your side. If not the vendor will control the process, not you.