I think get your questions
You are using a bushing-style single stage press and want to leave your dies set in the bushings for quick changes without readjusting on installation.
Sizing die - you don't need an extra.
Case-mouth belling yes.
Seat/crimp yes.
So, go get a used, tool steel die set at a gun show for a couple of dollars (there is little market for them since tungsten-carbide dies are so much more desirable) and put the steel sizing die aside. With the other two dies, there is no difference between a carbide set and a steel set.
4th die question? See this thread
http://thefiringline.com/forums/show...87#post5117287
But with a single stage press, there is no way I would not use the seat/crimp die and do my loading in the normal 3-die way.
Lost Sheep
EDIT: OK, not a single stage. See my next post