That is a hosted server, so you can't install anything you want on it: your hosting provider will manage operating system setup and configuration, and then you will put your applications (whatever they are) on it. So it's their right to choose which O.S. you are allowed to use, and if they are not completely insane, they are definitely not going to offer Windows 7 (or any other client O.S.) as an option.
If that was your server, you could install anything you wanted on it; but of course buying, installing and managing your server would have a completely different cost than renting a hosted one. And, even if that was the case, I'd anyway strongly advise you against loading a client O.S. on it. They are simply not designed for this kind of job.
That said, € 15 per month makes € 180 per year. As per this page, a single license of Windows Server 2008 R2 Standard Edition costs more than a thousand dollars, and a Windows server is very unlikely to keep using the same O.S. for more than five years, so this offer would cost you roughly the same as buying your own Windows Server license... but you will not have to manage it on your own. Which, as you "don't know much about OSes", may very well be a big plus.