A good site to buy pre-assembled PC?

Hi everyone, I should help my brother get a fixed PC, briefly he doesn't understand anything about PCs, I'm a computer scientist (programmer), I understand a little but I'm not an expert and more importantly we don't have the time to enjoy the most beautiful part of looking for and selecting the components, could you recommend a site to buy pre-assembled ones or could you give me some advice?

My brother would like to spend a maximum of €750, he needs the PC as a home PC for files, browsing online and playing a minimum of small games