If you're just buying a keyboard for osu, you should buy an osu keyboard or a numpad.
The actual brand of the input device doesn't matter nearly as much as the switch type in terms of raw performance. There are obviously additional things you can do, such as using PBT keycaps instead of ABS, leds, macro keys, and price.
Virtually ever mechanical keyboard is "good", or at least "good enough". In general, you will get what you pay for.
http://www.reddit.com/r/MechanicalKeybo ... ki/numpadsMost people prefer red cherry style switches. I prefer browns and topre.
scissor switches are also incredibly valuable to mania style play due to the singletap requirement.