I bought a RL550 back in 1993, before they changed the frame (now a RL550B). I couldn't begin to count the number of rounds that have been loaded on my machine. When I had my gunsmith shop, I would let customers use my press as long as they bought supplies from me. I have quite an assortment of pistol and rifle dies, all mounted in Dillon tool heads. It's a great machine, easy to master, and very productive. I amble along at 250 rounds per hour for pistol loads but you can crank out double that if you want.
To change from one caliber to another with the same primer size, all you have to do is remove and replace the shell holder plate, remove and replace the tool head (die pack), mount the powder measure and you're ready to go. This takes just a few minutes. Some folks buy a powder measure for each set of dies. I have one set up with the small baffle for pistol and another set up with a large baffle for rifle. You could easily get by with one.
When changing primer sizes, you have to remove the primer feed (2 screws and unhook a spring), replace the primer punch slide, then put the screws back in and connect the spring. Additionally, you have to unscrew a collar on the top of the primer body, remove / replace the primer tube and screw the collar back on. This complete process takes no more than 5 minutes.
I can do a worst-case conversion in less than 10 minuses. This would be from a small primer pistol cartridge to a large primer rifle cartridge. All parts required for small & large primers are included with the press as are the large and small powder baffles.
In addition to dies, you will need to buy a "caliber conversion kit" for each set of dies and a tool head (one comes with the press). The caliber conversion kit includes a shell plate, 3 brass guide pins, and a powder funnel (also performs the case mouth belling function). You can buy the parts separate to save money. Example: a 45 ACP uses the same shell plate as many rifle cartridges (308, 30-06, etc). In this case, you can buy just the powder funnel. Also, all 30 cal rifles powder dies are the same, as are .224 and 7mm etc.
Most any brand of 7/8X14 dies will work with the 550. I prefer Dillon pistol dies because they are so easy to take apart for cleaning and don't loose their adjustment. The only dies I had problems with were Lee. Some Lee calibers have short bodies that don't leave enough threads for a lock nut. Besides, using Lee dies on a Dillon is like putting a $25 scope on a $700 rifle.