If you are intent on purchasing the cheapest machine be prepared for trouble if your purchase does not work. Computers sellers are the new double glazing companies. After many years of chasing a bargain, I am now committed to purchasing from John Lewis when I need a base unit or laptop: products work as advertised, there is a 2 year back to base warranty (3 years for monitors; 12 months for laptops, I think). The price and range is not as great as the internet, but you get a product from a company with a respected record of customer service.
The stories on this forum (and elswhere ) about Mesh/Dell/Tiny and recently Overclockers makes me very nervous spending £1000 on a new system with these "computer household" names.
As contributors have suggested before, customer service should feature in PC Adviser reviews to help the unwary.