I am terrible with PCs and even worse with phones so bare with me.
I love the Nexus 5's UI but at the same time I want to get a Galaxy s5 is there any way that this is possible?
Just download the app Nova launcher, or apex launcher.
A launcher is your home screens and these apps replace them with more customizable ones. Then just search and download a theme called KitKat, or andoid 4.4 or Nexus 5. They are all the same thing. Go to Nova settings>theme>then apply the one you want.
Oh, and there are paid and free versions of the apps, even the themes so you don't need to pay for one unless you feel like supporting the developers.