Yes, swift can handle heterogenous drive sizes. For example, you can start with 2T drives and start adding 3T drives when they become more cost effective. You initial 40-60TB is well within reason for a swift cluster. I know of several running clusters at much larger scale than that (PBs of data, billions of objects).
Cost consideration are also highly dependent on your particular hardware choice. We recommend that you optimize your hardware for price per GB rather than for IOps or RAM or CPU. As you can imagine, there are nuances to all of this, too.
In case you haven't seen it yet, all the auto-generated docs for swift are at http://swift.openstack.org and much help can be found in #openstack on freenode IRC.
I'd recommend that you first look at the swift all-in-one docs (http://swift.openstack.org/development_saio.html) for running an entire cluster in a single VM. It should give you a good feel of the different parts of the system.