Mini-itx have some differences. Usually Mini-ITX boards are a laptop chipset instead of a desktop chipset. This can be good though because some newer desktop chipsets don't have integrated graphics. Sometimes the laptop chipsets will have a lower power consumption also.
As far as power, that Opus is more then enough usually. You may be able to get by with a PSU that pulls less watts. But be careful because some mini-itx boards have non-ATX compliant power connectors.
You should be fine with the CPU fan. If heat becomes a problem, you won't know it until it's installed. Then you can add some more cooling (better/more fans, new heatsink, better ventilation).