I started reloading .45 ACP on a Dillon 550b. I'm now loading .30-06 and .243 and I don't have any complaints.
Yes, I have to break out resizing/repriming from what would be considered a full progressive process, but after that it works as a "3-station progressive".
I suppose I can offer a few pointers -
1. You're going to have to do something I don't have to, which is change the primer mechanism to handle small primers. This isn't hard to do, but it does take a little bit of time. I'd take advantage of having things apart to clean things thoroughly. I do that once in a while, even though everything I load uses large primers.
2. You are going to need a different shell plate for .223, and when you change this part, make sure you get it adjusted for minimum play or you can wind up with "cocked" primers. The shell plate will allow the case to tip as the primer punch tries to seat the primer. Put a
small amount of oil on the little detent ball.
3. While the Dillon powder measure is excellent, it doesn't seem to like to throw extruded powders into bottleneck cases if they have any residue of lube in the neck. You can get powder bridging and have a real mess on your hands. You can try to use powders designed to meter well (like Varget), and you can use dry lubes for the case neck, like graphite or motor mica. If you use RCBS case lube, use just a trace on a case neck brush. I have occasional trouble with .243, and .223 is going to be just a little smaller so I can't see it being any easier.
4. Every once in a while, I go through a few hundred rounds at a time trying to feed a hungry Garand. While a powered trimmer like a Giraud seemed expensive at the time, I find it means I can trim as fast as the 550b can load. If you have plenty of time and are trying to save on equipment, a Giraud is not necessary. I'm still glad I got one instead of a single stage press, second powder measure, etc. (Different strokes for different folks, and there's nothing wrong with single stage presses. Except they're
slow...
)
Good luck.