So I'm a big fat liar. Right now I'm not going to tell you how to get carpet for free. Instead, warm up your singing voices because I'm going to start preaching to the choir.
Just because you want to do this on a budget doesn't mean it has to look cheap, and it doesn't mean that you have to sacrifice your vision for the project. Which means... you have to have a vision for your project. Imagine the color schemes you want and how you imagine each room to look (but be flexible). Originally, I wanted the bottom floor all in earth tones and warm colors, but then I found this gorgeous turquoise paint that I thought would give a very beachy feel to the room with the orangey-brown carpet. It's okay to make constructive compromises like that, but don't think you have to settle for the first cheap shade of paint that saunters your way if it really doesn't fit.
Imagine a purpose for each room. That'll help you get a clearer picture of what you want. Personally, I want to work in a spaceship room somewhere. I'm not going to make the house representative of any time period in particular, because I like the idea of a hodgepodge, the way a modern house can be hodgepodged. Also because I'm not a woodworker. All the woodwork I do will be able to be replicated with an Exacto knife and a pocket knife.
If there's something you really want to do, you can find a way to make it work. Happy arting!