AHCI -- yes
Creating a partition for your OS in roughly the same size as your anticipated new SSD may facilitate an easier transition.
At a minimum, you should create several *blind partitions* (at minimal size without setting a drive letter) for any unanticipated future need.
By nature of a mechanical drive, some portions of the platters will spin faster, but I've never tested a 2TB drive for optimal partition sizes - and in the big scheme of things, including your future move to an SSD ...
I wouldn't worry about it!