This is a community dedicated to building custom computers of all shapes, sizes and performance classes, mostly from off-the-self parts, but custom fabrication is of course also welcome.
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Community icon by Abderraouf omara, licensed under CC BY 3.0
Depends how hard you plan to make it work, and if you intend to overclock it.
I recommend just building it then seeing how hot it actually gets. Realistically you'll probably just find the fans all turn faster (making more noise) and that you should clean the dust out a bit more often
You're fine. The fan in the back is the only one getting air out of the case, so having 2 fans with "not great" airflow is not really gonna make a difference. An inch of clearance is generally the minimum, think of how your power supply can get air from the bottom when it's only raised an inch from the floor.