Is it possible to install unmodified, unadulterated and pure Android on a Sony Xperia S? I'm willing to buy a Sony Xperia S, but in my opinion, the modifications Sony made to Android is just horrible.

So, is it possible to install the last version of Android unmodified? And, if so, how do I do that?
Of course you can

Just pick one from all the available ROMS, you will find some vanilla builds there if you only want the basics. But it requires the knowledge of rooting your ROM and flashing

if you are afraid of flashing but not rooting, then you can just remove the apps that you don't want using Titanium backup.