250k hits is a fair number. You'll need more than IIS or Apache to make it work - mainly expertise. Anyone can bring up a webserver and run it, but to run it well you need the experience of all its foibles (and you'll think IIS has more than most if you come from a *nix background).
eg. Slashdot serves 4 million pages per month, and they have a large infrastructure. You're looking to be a fifth the size! So whilst IIS can do it, the bigger question is whether you can do it. You'll need to learn a lot of brand new technologies and chances are your solution will not be as good as the stuff you're used to, it'll also take you a lot longer.
So, if I was a businessman selling this, I'd try to make them change their minds - show them your existing customer sites, example data from large systems, inform them of the risks in a new platform. Its not as if you can't do the work in a non-.NET toolset, its not as if there are very few web devs who don't know .NET, there's little reason to go with it, in this case.