You do not need mach2, mach3 has all the functions of 2, and much more.
A 'post processor' is part of a cam program that generates the code for one specific machine. Sheetcam has many posts, you need the one that generates 2D code.
What les suggested is to use the Mach2 2D post processor with sheetcam. It could just as well be called a Mach3 2D post. The 2D part says sheetcam will never call for a Z move- it will put an M01 in the code, which will cause mach3 to stop and wait for you to press Start. You would manually operate the Z, to whatever depth is called for, then hit start to continue the program.
If you try to run a program with a Z move, and Mach is not setup for Z acis, it will hang, as youhave discovered. You should not do that- you are asking Mach to do a Z move that it knows it cannot do.