There's not much that hasn't been said.
I used to sell presses (Dillon's) and they advice I gave then still applies.
If you know you will end up with a 1050 eventually, don't waste your time and buy it now (if you can afford it). No point in going through 2-3 presses just to end up with the Cadillac. If you are loading one or two calibers 98% of the time, either the 1050 or 650 will be great. I say that because the caliber conversion are more time consuming on these two machines, especially if you need to switch primer sizes. Also, the 1050 is the only Dillon press not covered by the lifetime warranty as they deem it a commercial machine.
If however, you are loading multiple calibers, and output isn't your biggest concern, a 550 may very well be all you need. No, it isn't auto-indexing but you can still easily
roll out 500+ rounds an hour. That's without the case feeder, which is more gimmicky for the 550 as the press was never designed for one.
No matter what press you buy, buy quick conversions as mentioned and lots of extra primer tubes. The strong mount is a nice to have, as is the bullet tray and roller handle.
Dillon's warranty is legendary. I had someone come into the store once with a mangled 20 year old Dillon they bought on Kijiji. It was beyond repair, but we sent the whole thing in to Dillon anyway. Not surprisingly, a brand spanking new Dillon 550 was sent free of charge as a replacement within a week.
I have a feeling you already know what you want to buy, you're just needing that extra little push to spend the money on a higher end Dillon