Should I buy a single machine or a complete line?

It depends on where your bottleneck is. If you already have a working line and one machine fails, replacing that single machine is correct. If you are building new capacity, a complete line is usually the better investment because the interfaces - conveying, batching, dust control and control logic - are where performance is won or lost. A single granulator cannot fix an upstream batching problem, and a single bagger cannot fix downstream segregation. Tell us your current setup and target output; we will recommend the smallest complete scope that solves the problem.