I find the Flex to be a no brainer really, I have a PC XP and I had a Flex and I sold it as I wasn't doing much detailing but now I realized I made mistake. The Flex is stronger machine and it cuts down on time, If you are trying to get a swirled/scratched up paint to near perfection I don't see how you can do it with the PC and finish in one day, especially if you're doing the interior too (full package). Go with the Flex and never sell it under no circumstance.
As for the tight area I would try the focus pads and if that doesn't work in some very tight areas, like behind the door knobs where you knuckles hit, I would try some Fine Machine Polish on an applicator and do it by hand, I have great results on a C300 I just detailed using this method.
Enjoy your Flex