I've posted this in answer to the question that the OP sent me by PM.
Yes you can deprime with the collet die and seat with the seater die. The collet die also is where as well as depriming the neck sizing...and neck sizing only...will get done. But do you need a full length sizing die as well? In my opinion yes.
That is because eventually after maybe three or four firings you may want to full length size your cases particularly if you rifle has a sloppy chamber. Perhaps calling a chamber "sloppy" is a bit incorrect. What is more correct is a chamber that is perhaps a the larger end of the permitted tolerances.
That's good as it means it will accept more ammunition than a rifle that has a tight chamber (more correctly one that us at the smaller end of permitted tolerances) but the price is that obviously the cases when fired will expand more in the so called sloppy chamber.
As a case in point at one time I had two identical of the same model P-Hale .270 M81 Classic rifles. Cartridges fired in rifle A if neck sized (such as with a Lee Collet Die) and loaded would chamber and fire in rifle A and rifle B.
But cartridges fired in rifle B if neck sized (such as with a Lee Collet Die) and loaded would NOT chamber and fire in rifle A. They'd chamber and fire in rifle B no problem. All day long. But not in rifle A.
This was as rifle B had what I've termed a sloppy chamber the expansion of the case body (which a Lee Collet Die won't resize) was such that if neck sized only they'd not chamber in the tight chamber of rifle A. Yet in the two rifles BOTH chambers were in accepted tolerances I'd add. Just that one was sloppy as I'd term it.
So the bottom line is that whilst for three or four loadings you will be OK with a Lee Collet Die it is always sense to have a standard full length sizing die in the back of your cupboard for use after that fourth loading or if acquiring someone else's once fired, or fired, cases.