If you can get it for cheap, go for it. I had a 450. You can't believe what a PITA changing calibers is compared to a 550 with the tool heads. 20-30 minutes of pure frustration (on a good day) versus 2 minutes. If you can dedicate it to one caliber, do that. If not, get the conversion that's listed in the Dillon catalog. You can make the 450 into a 550. Trust me on this.
For multiple calibers....if I could get the 450 free or pay retail for the 550, I'd take the 550 hands down.
You said you load 1k at a time. Once you understand what a progressive will do, 1k is nothing. Sometimes I go to the garage/ reloading room when I'm bored. You can sit down and knock out 2-300 in less than an hour and go on to something else.