People tend to build their own machines from scratch rather than buying a particular machine which is why I suggested having a read through the build logs. If it is designed in the right way then you could add a 4th axis at a latter date if required. Not so sure about using all the other main parts though, motors & drivers shouldn't be a problem but re using the linear rails & ballscrews wouldn't really be an option. If your increased table size were limited to the original length then those rails & ballscrews could be used again but the shorter ones would probably need to be used on another project or sold on.