I am guessing you already have the right shell plate and pins for the caliber you plan to load. It would be best to have the through die expander for the proper caliber to go with the Dillon powder measure. The Dillon powder measure is quite good. The only problem I see with Lee dies on the 550b is you might have to go a little slower using them. They don't have the radius of the Dillon dies. That difference lets the Dillon self correct better when you go faster. When using the Lee 4 dies set you would then not need to expander die since it will be part of the power measure die setup.
I hope that helps.