Just an update on this - in the end we went for the Fractal Design Core 1100, Asrock B450 Pro4, Ryzen 5 3400G, 16GB of Corsair RAM and a Be Quiet 400W semi-modular PSU.
I put it all together last night and bar for a hairy 10 minutes trying to get the motherboard in without snapping it, all went to plan and it seems to be just the job!
Thanks for all of the suggestions and advice!