What are you more interested in, riding or restoring? If you primarily want to ride, you are going to be much more satisfied with a new bike or a newish used bike. Because technology trickles down, a cheap modern BMX bikes is going to function better than an old BMX bike, and is probably going to cost you the same or less than a restoration.
On the other hand, if you are more into the idea of restoration, first look into what model of frame you have to make sure its worth dumping more resources into.