It’s not a myth to say that you get what you pay for. Buying cheap painting implements will produce a cheap looking finish. In particular, it’s worth investing in some good quality brushes.
Always buy a reputable paint brands. There may be cheaper paint on offer but they will probably not cover so well and the colour may not last.
If you are painting the walls and ceilings then invest in an extending roller pole rather than using a ladder. It’s much faster and less strain on your back and arms.
Powder filler enables you to mix up the exact amount at the consistency required. It’s also less likely to go off, particularly if you keep it airtight.