I was just going for air cooling using the fan with the processor in the first instance. Currently I have a geforce 980 which is around 30cm though I'll likely upgrade to a 3000 series card when they launch later this year. CPU wise I was considering a Ryzen5 or maybe the upcoming Ryzen3 depending on pricing/performance.
Case wise I like the look of the NZXT 200i though maybe you're right in saying I should consider M-ATX instead of M-ITX. Would I be able to go for a substantially small case and still fit a high end GPU if I went for a smaller case with a non-ATX PSU?
Budget wise I was thinking of around £100 for the case. I have a spare ATX PSU which is why I'd wanted a case that could fit it.
At £100, here's a selection of options to get you started. See what you like:
ATX PSU:
NZXT H200/H210 (ITX tower)
Inwin 301 (small Micro ATX tower)
Raijintek Ophion (ITX sandwich case)
Silverstone SG13 (small cube case)
Fractal Design Node 202 (console-style case)
Cougar QBX (short ITX "tower")
SFX PSU:
Lian Li TU150 (similar design to the H200, but smaller)
Phanteks Evolv Shift/Shift Air (unique tall tower design)
Raijintek Metis Plus (really small ITX tower. Can take an ATX PSU, but at the expense of GPU clearance).
TBH, at this price point there isn't much that requires an SFX PSU. Closer to £200 and you start to see it crop up as a requirement more often.
If you can pick ones you like from the selection above, it'll give people more of a starting point for recommendations. ITX cases come in quite a wide variety of form factors.