I would also see what kind of ASPX stuff they may want to be running. Are there PHP equivalents out there?
I had a client that wanted to move their site over, but they had an employee area written in .asp. However, what it was was essentially a document sharing area and a group calendar. I ended up introducing them to Google Apps, which ended up being much better than what they had already.
Anyways, my point is... Either you could find a way to get their aspx stuff to work, or there might be something else they could look at using, too.