I think a lot of us preach about starting with a single stage press because that's the way we started out and it has served us well throughout the years. I actually see no problem with starting out with a progressive and the Dillon RL 550 B is one of the best choices out there. However, an investment in a decent single stage press would not be a waste. About the only items necessary for the use of the single stage press (aside from the press itself) that wouldn't be needed for use with the Dillon would be a powder measure, shell holders and a loading block. You will always find a use for the single stage in your reloading. When working up a new load, you normally only want to load 10-20 rounds for testing. This is much easier to do on a single stage press and if you get into accuracy loading for your rifles, the single stage press can give you much greater control over the process especially when it comes to weighing individual charges.
In the final analysis, what's most important is your paying attention to detail. With a progressive, it's sometimes easy to get "lazy" and not pay as much attention as you should. Learning on a single stage creates good reloading habits that will carry over to your use of a progressive.
Good luck whatever your decision. Reloading is a beneficial and rewarding hobby that can often become as enjoyable as the shooting itself.
ps -- you may want to do a search on reloading for the Glock 40. I don't own a Glock, but I've read several articles that indicates they aren't the best candidate for reloading due to not fully supporting the cartridge case in the chamber.