any SFF (small form factor) Barbone model produced by Shuttle will be great. There is a big user base (including myself) out there help each other.
Finding info for different models click here
User forum click here
There are many suppliers in the UK, the one I like click here.
The model I like for AMD CPU is SN45G, if you like on-board graphics SN41G2.
All can be modified to make very quiet.
But the quietest one (very few other in the world can beat it yet) is ST62K which use Socket 478 intel CPU. Best to get Northwood CPU because Prescott CPU is very hot.