PaulieGB,
As many others here have suggested, I would go for active speakers as you can avoid a lot of headache chasing the perfect matching power amplifier (whatever that means) and speaker cables - when you buy an active speaker, you get a perfectly tuned all in one setup with the inherent superiority of an active XO. I would spend the majority of your money on the speakers.
As for the rest, I'd really just buy a competent pre amp that has the features you need and move on. If you only use digital sources, then obviously a PreDAC like the Benchmark DAC2 might appeal, or a Linn DSM, or any preamplifier with a digital input - your actual digital source can then be something quite cheap like a Sonos or a squeeze box.
You seem to have set your mind on ATCs and owning some myself, I am not going to argue with you. Two things:
1.) you seem to have had a demo at ATC themselves? It is unfortunate that they only had the 25 and the 150 on display, and nothing in between - choosing between the 50, 100 and 150 is dependent on your room size and how loud you listen; I have 100s in a 22.5m2 room, but I rarely listen very loud so I get away with it
2.) the 50/100/150 all have a slightly better mid range driver and (I think) a slightly better tweeter than the 25; also ask about the forthcoming tweeter which is designed by ATC themselves and should be retrofittable to the 50/100/150, but not to the 25s, due to space issues
Best of luck with your quest!
Samuel.