DId you buy the 550? or are you just considering it?
Is it complete?
I had an RL 450 same system as the later 550 just without the removable tool head.
IF the press you are looking at has the Dillon powder measure, why waste it by buying Lee??
It is, however, a bit tricksy getting it fully and properly adjusted. Dillon powder measure, designed to work with the Dillon "powder through" Expander die. This means their die body, their expander/powderfunnel and their powdermeasure all together. Change parts with something from another maker and its a good bet that even if it fits you won't get correct adjustment.
So, no, don't stick your Dillon measure on top of the Lee die or Lee measure on the Dillon die, its not going to work right, if it works at all.
The other die stations can use anyone's standard dies, resize, seat, crimp any Lee Lyman RCBS etc will do fine, but not the expander /powder drop station.
Look at the way it works. (also are you looking at loading pistol or rifle??)
For straight wall cases (pistol primarily) the powder drop tube is ALSO the inside neck expander and case mouth flare. So, for a pistol case, you run the case into the die, the neck expander tip of the drop tube enters the case mouth, and gets LIFTED as you raise the ram.
This lifting of the drop tube operates the powder measure and proper adjustment is that when the measure is fully "dumped" (cavity all the way open) this stops the tube from going any higher, and THEN the last bit of ram travel pushes the case up a bit more onto the now stationary drop tube, completing inside case expansion as the powder is dumped into the case.
Now for a bottle necked case, the process is similar but slightly different and the difference is in the adjustments of the die body location.
So, for a bottle neck case the powder drop tube does not do inside neck sizing or mouth flare, it is just a tube made to fit against the case mouth. Ram goes up, tube is pushed up by case mouth, but this time, you want all the ram travel at the same point as full powder measure cavity travel.
This is how it works with the Dillon parts I have no idea what it would do with Lee parts, sorry. If you have the Dillon parts with the press, use them. Learn how to adjust them correctly and they will work. Don't get it right, and there will be issues.
ALSO, keep an eye on the powder measure, see that it cycles all the way both ways when operated. They can stick.