Hi Arda,
If you want to upgrade later, you might have to pay a $150,- extra to have the slots ground in for the pc control to hang in to.
If you order a PC straight away, I am not sure if the costs come extra or if they are service.
Best ask Steve about that.
I would suggest that you get the slots milled in whenn you get a classic.
If you would like to upgrade later, you would not have to send your classic back for the slots.