If you want one that is very accurate, get a Moore and Wright square. These will be more than sufficient for engineering purposes.
As you will be using it to set up woodworking machines then companies like RDG Tools sell good quality squares.
It is important to know that a square may not be accurate on both it's inner and outer edge, usually just the outer edge, so if it is important that both edges are accurate, check the Conformance Certificate.
I would avoid buying a second hand square as if they are dropped it can knock the accuracy.
