I knew from the beginning that I wanted to build my own case.
Regarding the power supply, emphatically yes, you have to buy a reliable source with plenty of energy.
When I started researching ahead of my own leap into modular four months ago, the repeat guidance in countless threads to start with a Mantis case was clear and convincing - four months later and over half full, I think it remains the best tip I took on board. It might not be the most exciting part of the journey, but with so many other things to learn and navigate, keeping the case and power supply simple, reliable and reasonably cost effective would be my advice too.
-- alexandershawkins