I was a complete newb and bought a 650 a couple years ago. Didn't get help from anyone to setup. Go slow, watch lots of you-tube videos and read the manuals, invest in a powder check. Buy an inertia hammer to take apart mistakes or rounds that doesn't size check. Watch the Dillon Case Gauge video and explanation about the step on their gauge. Read Dillon's troubleshooting help and videos prior to anything - so you no what goes wrong and how to avoid it.
When you are starting out and making adjustments or tweaks to the press, check your rounds frequently. Check about every 5 out of 20 until you get a feel for what the is going on each time you make an adjustment. If you are a technically minded or, mechanically minded person you will be fine. Just put in the time to read and learn the basics, particularly safety. Once you get it figured out you don't need to check as often. I take the time to check a few every 100 rounds.
650 vs 550? I went with the 650 because of the 5th position. It allows you to use a powder check easily, and also grow into future add-ons, if you ever decide to get them (bullet feeder). With the 550, you don't have as much flexibility, but there is a way for additions.
If you haven't heard of them yet, check-out inline fabrication. They make some high quality mounts and press/tool change systems.
I splurged from day 1, recognizing that Dillon products seem to hold their value well on resale and have a lifetime warranty for most items.
Powder spill a warning: 9mm is a round that tends to be easier to spill powder out of on a progressive due to high fill level and no neck. If you spill, stop. completely clean the press. I found rushed powder clean-ups from a spill result in powder working its way into moving parts and causing more discrepancies in the overall lengths of rounds. Supposedly, +/- 0.003 inch is "typical" according to Dillon.