Having been through your process about a year ago (USG, UC-CK, 3*US-8-60W, 3*AP-AC Lite, 1*UAP-AC-M, 1*Netgear GS308) I'd consider the following:
Have a look at the Unifi Switch 8 US‑8‑60W switches instead of the US-8-150W's, they run 4 POE points, which looks like it might be enough on at least one of your switches, and cost about half the price.
Very good call, I've updated my original plan to go for the 60w switches and this shaves off almost £300.
I'll stick with the Pro APs though as I want the 3x3 radios, high bandwidth and high TX power... I fully intend to allow my future car to update systems by merely being parked outside, and I shift large files around my internal network on WiFi so that would be beneficial too.
On the Cloud Key... I prefer the form factor of the gen 2 and the little info screen, and the fact that it's a bit more powerful. And am hoping that the gen 2 just means a longer life too.
So thank you... I've got it to a price I'm willing to accept, and will pull the trigger.