I'm guessing you can pick up USB Bluetooth adapters new/retail for about $30-$40 now. To have a Mini configured with one is $50. If it were me, and I didn't have three lying around, along with a USB adapter, I'd probably go for configuring the mini with BT built-in.
... my point exactly! I've got them already, so there's no real point of having it built in and paying extra.
One other thing. Will my wifi dongle work with the airport express thingy that apple do?