You should save the $400 and continue to add to it. Your room is normal size. My room is the same size. Would I prefer to have a larger room? Absolutely, but we’ve down sized due to retirement. I’d recommend acquiring a system to fit your room rather than simply building a system that doesn’t fit your room.
You can acquire lots of used gear, many at excellent prices if you diligently search. But for me I’d concentrate on saving in order to acquire gear that’s worthy of your room.