You shouldn't go too cheap on a power supply, everything is dependent on it. There are plenty of well manufactured Bronze rated supplies, that just lets you know how efficient they are. Saves a few dollars on the electrical bill.
XFX 550W is a popular choice. Seasonic, Antec, Superflower, and the more expensive EVGA and Corsair are made by good manufacturers.
Overclocking power ratings are difficult to measure, depends on the configuration of your system. Even then it varies by the individual chips (CPU/GPU/RAM) you receive and what voltages you have to run through to get the desired results.
Haswell chips nearly double their TDP on a high overclock. GPUs vary wildly, but you can often go by the max value of the input to the cards (Slots 75W, 6pin 75W, 8pin 150W. So a GTX 780 Ti or R290 'could' pull a maximum of 300W. Again those are maximum estimates, the wires used are generally underrated.
Best thing is to find benchmarks from similar builds on this website and places like overclock.net to get real world values.