If you're buying new from Dell, you can customize to have the internal BT
installed...buying from the outlet or ebay, it isn't always apparent whether there is an internal BT
module or not.
If you end up getting a Mini with no BT
installed, it is relatively easy to install, but you will have to disassemble part of your Mini to install it. Your best bet would be to order the module directly from Dell, via email or chat, since they don't sell it on their website. There is a good step-by-step instruction HERE
on how to go about doing it.