Hmm...I'll probably get some disagreement here, but what the heck. First, let me state that I do NOT have a 650. I do have a Dillon 550, and love it. From what I have heard and read, swapping calibers on the 650 is a pain, and unless you're going to run 1000+ round batches, not worth the hassle over going with a RL550b. Once again, this is strictly second hand info. If nothing else, maybe someone with a 650 will dispute this.
Either the 650 or 500 will be a great press.
As far as accessories, here's a good starter list.
Digital scale - I have the Dillon and love it.
A good dial or digital caliper.
Quick Change kits for every caliber you plan to load.
Carbide dies for the straight walled pistol cases.
A tumbler and media seperator
A bottle of Dillon Rapid Polish - Really shines up the brass.
At least on more reloading manual. "Metallic Cartridge Reloading" seems to have more loads of different powders.
That's about all I can think of right now.
Welcome to the hobby,