Hank makes a very good point: The 550 will be as accurate as anything out there. I have verified this with some very sturdy presses (ie ammomaster).
The only Lee I'd recommend is one of their 'O' type presses. It has the advantage of being able to accomidate 1 1/4" dies and has a neat spent-primer tube setup with a hose that works 'most' the time.
The single stages allow you to use Redding's precission shellholders for 'bumping the shoulder' if the 223 is only used in one rifle. This headspace adjustment is very easy with those shellholders but can can also be done on the 550 as well.
First get some Hornady Sure-Lock Rings. These grip the threads of the die. Making your headspace adjustment is a simple matter of turning the ring instead of the die now and tightening it in the toolhead. Makes things MUCH easier. Another alternative to this is to get Sinclairs Die Shims which will do the same thing, but you'll need the Hornady rings with those as well. I only use those rings on my body dies... but is the poor mans version of a lock-and-load system. Use them and your dies will be placed in the same position every time if they need to be removed.
For my money, I'd get another toolhead, some lock rings and some die shims. If you don't care about headspace and full length sizing, then just get a toolhead.
For my shouldered rounds, like the 223, I use a three-phased process on my 550.
First I tumble, lube (dillons spray, what else?), decap, size and neck size on the dillon (three dies).
Then I tumble to remove the lube, trim to length, clean primer pockets and inside necks.
Then it's back to the Dillon for primer seating (decapper die in stage 1), charging and bullet seating.
That's a lot, I know, but it's is important you trim to length after sizing.