You don't get the original WoW with TBC.

If you're not going to start either a Draenai or a Blood Elf, you can postpone buying TBC until your lot closes in on the mid-50's, when it's worth getting your team portaled over to Outland.

You can upgrade your account to TBC instantly, online, and since you already have the TBC disks you won't need to download the data when you make the decision.