Well I do want to the overclock in the future but is SLI/Crossfire needed in games such as Battlefield 3, Skyrim or Mass Effect 3?
If you don't overclock you can actually get a system that is more powerful as there will be more money for the GPU, the cheapest K series is the 2500K which is £160 which is more than a quarter of your budget.
eBay is not a problem for me as I only buy from eBay's top sellers (assuming that they're not frauds). Although is manufacturer warranty still on the item if you buy from eBay? I would of bought components from eBay by now if I just knew if there was.
Even then it's iffy, it depends on the part. I always ask for a receipt of the part (when they bought it) in case of RMA requirements. Haven't had to do that yet so can't say what would happen but i think it'd be okay.