Hi, Hakan-Sukur and welcome
The M6F is a great choice, but check out the Maximus VII being announced this week. An M7F may be more attractive for a new build at this time.
All-in-one watercooling kits are okay for a CPU. I have not seen any AOI kit with enough pump to add any other components to the cooling loop. They all come with one waterblock for the CPU and do not expect anything else to be added. Many of the AOI use a single-fan radiator. Those are acceptable for a moderately overclocked CPU, but water temperature would rise if more power, for instance from the VRM (power regulators), were added.
A plan to add anything to the cooling loop has to consider a custom loop. That can easily be extended to the VRM of the Formula and even to DRAM and video cards. I include only the VRM at the moment.
Watercooling performance depends strongly on the rate of water flow through the CPU block and the radiator. 1/4 inch tubing restricts water flow compared to 3/8 diameter tubing which is strongly preferred as soon as you go beyond an AOI. When several components are cooled in a series loop, they should all be connected with tubing that doesn't restrict water flow, eg. 3/8. Tubing and fittings are available in 3/8 diameter at virtually the same cost as 1/4".