Which M/B would be recommended - there are so many to choose from.
I would recommend to go with the same mobo the missingremote review Intel Core i3-2100T and DH67CF Mini-ITX Motherboard
, specially since you want to go with picoPSU, this mobo comes with the PSU connector away from the ram slots.
Would I get away with a picpsu (120w) with a regulated 95w external power supply? Or if not, can anybody recommend a quiet atx PSU?
My recent build, idles at 14W and loads at 50W, without a dedicated GPU. The AMD/ATI "specs the 5450 at a mere 19.1W TDP, and an idle power usage of 6.4W", so it should be fine, but i personally went with the 150XT as it comes with 4pin power and more room to grown later, but you should check the 12V amp on the pico you chose and the your complete setup to see if you will be inline with what the manufactures asks. If you go with normal ATX PSU, consider Seasonic X400 fanless
Can anybody recommend a passive or if not a quiet low profile CPU cooler?
The intel 2100T comes with a very low profile stock cooler
But if you want to go with aftermarket low profile cooler, check Scythe Kozuti Low Profile (40mm Height)
(but cross check the mobo and placement on your case to see if it fits). The intel mobo has a 4pin pwm fan conector, it will lower the rpm a lot, in my case stock it lowered the TY140 to 700RPM and the Kama flex 120mm to 300rpm.
But if you want to go fanless, you probably should check HDPLEX H3.SODD Fanless Mini-ITX HTPC Case
or the H5.S FANLESS CHASSIS
to use your dedicated GPU.