I don't own the Flex so I can neither support it or denigrate it.
I do own a PCXP. Based on the feedback from forum members, the Griot ROP is more powerful than the PCXP but I'm not planning on replacing my PCXP with a Griot. You can buy either one and have a quality tool. If you buy either machine, buy 5.5" pads with a 5" backing plate for major surfaces. Many of us have invested in 4" pads and 3.5" backing plates for our DAs too.
The Flex is more powerful and if you are doing many cars or every damaged paint, the Flex offers some advantages. It seems that many of the guys with a Flex also own and use either a Griots ROP or a PCXP. You may find that you will want both and you may find that you can't do everything you want with the Flex and will end up doing sections by hand if you don't have one of the other DAs.
I have Surbufs at home but didn't get the two cars that I planned to test them on over Christmas. The Surbufs may offer the cutting capability of a rotary with a Griots or PCXP, so the Flex might not have the advantage you expect especially if you are only doing a couple cars and not really damaged or difficult paint.
Don't get me wrong, the guys with Flex's love them. However, I'm pretty sure that most have more than the Flex for situations. If you can afford it and plan to get "involved" in detailing, buy both a Flex and a DA.
Buy good Microfiber towels. You have lots of choices. The Cobra edgeless MF towels are very good towels, however their supplier recently sent a shipment that was not produced as spec'd. Give them a chance to bring in a new lot that meets their expectations and I will then recommend them fully. I personally will hold off until that the proper towels are stocked again.
ONR is a pretty good clay lube, but you have a lot of choices. If you have a Mother's or Meguiar's clay kit, you'll be fine for basic claying. They both include adequate clay and clay lube for a single car. I might not use it on a $500K exotic now but my daily driver BMW's were fine for the years that I used them.
I may be crazy but I really like Optimum Power Clean as an APC. Yeah, it's a little more expensive however it works great in many situations and seems safer than some other cleaner/degreasers.
I also like Wolfgang DGPS3.0 for a sealant. The Megs twins (105/205) are a good combo with cut and finish with a DA.
Good Luck and Welcome!