Standard Dillon configuration is
Station 1 - Decaping/Sizing Die
Station 2 - Powder Die
Station 3 - Seating Die
Station 4 - Crimping Die
The Dillon dies are very nice and designed for use on progressive presses, and are fully supported by Dillon. I would use the Dillon dies unless you have a specific specific reason for going with something else (i.e. the Factory Crimp Die which is a better mouse trap). The RCBS dies are probably not a better mouse trap. The Seater/crimper you mentioned will be difficult to adjust and just double up (or work against) the work the Factory Crimp die does. Seperating the seating and crimping stages is a good thing.
The difference in cost between the die sets should be irrelevent given the savings on ammo.