is your brother looking to play games with it? or just browse the net, email and write documents, etc? if the latter, then you can build an AMD system for much cheaper than 300 pounds (american probably $450-$500?). gaming is a whole other story. ever check out pcpartpicker.com? great site.
as for the amd being ~ the same performance as intel and nvidia....yes and no. in the budget department, that is probably fairly correct. but i wouldn't base your build on AMD just because they're cheaper. dual-core intel i3 CPUs definitely hold their own against high core-count amd CPUs....in gaming. multi-threaded rendering and such software that can take advantage of more cores....amd seem to have the slight edge there in the budget department. just go with the best performace/price ratio and try to ignore anything the AMD/intel/nvidia fanboys say.
if you ARE looking to do some gaming...i would base your "budget" build around the best GPU you can afford. something maybe about 1/3 - 1/2 the total price of the build. this being around 300 pounds, i personally would go with something no less than an nvidia gtx 750ti (~80ish pounds). if you're not concerned about efficiency, an AMD r9 270 (not 270x) would be a very nice gaming GPU for around 90-100 pounds. disregard the r9 265 since its about the same price as the 270 but, obviously, not as fast. you will definitely be happy with any of those choices, but i wouldn't expect to be running ALL games at 1080p at full details.
hope i may have helped you a little bit, at least? cheers!
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