As I understand it, Steam Machines are not particularly powerful so I think it would be worth exploring pre-builts that are around what you want to spend to see if you can get something comparable or better. SteamOS will be available to install on all desktops at some point but in the meantime there are plenty of gaming focussed distros that will do a superb job. Unfortunately with memory prices the way they are you aren’t really getting the same bang for your buck that you did previously, but there isn’t much any of us can do about it at present. Good luck with it all, it is certainly a very exciting time to be getting into Linux gaming!