What "enterprise software" is needed at a public administration? Almost all of it is custom ordered, so Linux compatibility can be a part of the requirements.

Outlook is a dumpster fire, and there are perfectly fine alternatives. Very few people actually need the full might of Excel, and you can always RemoteApp it to them, and have the rest use Libre Office. Or heck, provide them with actual tooling better fit for their requirements than Excel.

And in any case, IMHO, "public money, public code". Nothing should be paid for with public money without it being publicly accessible afterwards ( excluding weaponry of course).