About Power Supply

HEC or Task would be a good choice.

most of them are made on same circuit design and need not to worry about brand. whatever you supplier supplies is ok and check for warranty coverage and period thats all.

All brand is Okay, check for the recommended voltages to give sufficient energy for stability. Many new processor need bigger power than usual processor. It's enough for me to use 400watt if my computer is lower than Pentium 4. In fact, every brand have same component if you see how people create a tool.

Brand doesn't matter. Just make sure that 400 is enough wattage for your computer.

While buying a power supply see those specification such as efficiency(it would be very high-maximum is 100% you cannot find 100% above 98% is good), power consumption, ability to withstand fluctuations (see maximum and minimum voltage required) and also the noise factor.(It should be as low as possible). Also, brand doesn't matter. Try to see the specifications before buying it. Also, try to know whether it suits your mother board. Because if your computer is too old, it may not have the ATX motherboard, it may have AT mother board. So also note it too. Have a great day.