Rust is a fairly un-optimized game at the moment. Even some top tier gaming machines have trouble running this at ultra. The developers are currently working on game optimization, which will be implemented (apparently) in the next 2-3 updates. With your budget you would be able to afford a fairly low end gaming machine, it will most likely be able to run the optimized version of rust on low and just be able to cope. In the state that rust is at the moment, it wouldn't be able to handle rust even on the lowest settings at ~60 fps
Edit: Regardless, the new rig would be able to run rust better than the macbook. If you would like more help with components feel free to PM me.