All of the suggested accessories are, in fact, very useful, but it depends on what you end up mostly using the MFT for. I've bought dogs, Rip Dogs, Festool quick and screw clamps, Festool clamping elements, and other accessories. I've found some to be a waste of my money for what I mostly do. The accessories I do use a lot are the Screw Clamps, Quick Clamps, Clamping Elements (without the anchoring knobs) since you almost always need to hold things in place while assembling or Dominoing or routing. I use my MFT3 a lot for crosscutting with the fence/guide rail and, for this, I usually don't need anything else. For sanding, I mostly use the clamps or clamping elements.
I'd recommend that you wait to buy a lot of add-ons until you find out what you really do with the table and what is lacking when you use it. Could save you some money.