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I'm sure whether the rad falls apart or not must be down to the quality of the Rad you bought it the first place mate, I'm not saying don't buy a big KOYO rad I did and it worked great but I don't think it's really necessary is my point, You have to look at controlling the engine temps as a whole not just the water, So AIR/WATER & OIL temps need to be looked at as a whole package.
It also depends what you intend to use the car for, If it's just a fast road car then I don't think you need to do much more than fit a good quality metal rad, And only because the plastic crap Nissan used just wasn't much use anyway, It also depends on how much boost and power your car has, Making room for the Koyo rad isn't a five min job so think about what you want first.
All I can say is I've never he heard of a Cooper rad falling apart before, But they're cheap enough anyway. If you buy one from someone like AbsoluteRadiator.com in the states I'm sure that the quality will not be an issue, Just email them and say you've heard about them falling apart and see what they say.
Rob