So my question is this: Is there any real difference in the regular seating die (with the crimp function backed off so as not to crimp) and the DL seating die? Any advantages or disadvantages of one over the other?
The supposed advantage to the dead length seating dies is that you can set them up to contact the shell holder, so any flex or play that may be in your press won't affect the seating depth. Personally, I haven't found it to be a big deal, but I don't use a turret.
I agree with m&p45acp10+1 that the expander die shouldn't be necessary unless you're loading cast bullets. From your description and pic, that doesn't look to be the case, so no reason to spend that money.
As far as the crimping question, I think everyone agrees that crimping with the seating die is not a good option, which is what m&p45acp10+1's experience is with. Crimping with a FCD is completely different, and a subject of some controversy. There's been a thread about it over the past few days, read that if you want more info.