The R7-250 itself doesn't require any external 6-pin connectors, so unless you plan on overclocking it to pluto and back with liquid nitrogen, I'd think a 450-500W power supply is plentiful. (Antec VP450. Do it. It's rated at 450W, but pushes 550W no problem with little to no extra heat produced. Unless SLI/XFire, then...)
As for which brand, I'd always say pick the cheaper one, but the brand war will rage on forever. Some swear by Sapphire, others swear by Asus. I just get whatever works best for the least money. My poor little Asus r7-240 (2gb ddr3) was overclocked to all hell and beyond, (1017mhz core, 1325mhz mem) and it was more stable than I ever thought it would be.