Mason Dixon;
It has been a long time since I hooked up a Mach3 system parallel port, but I believe you can use some of the PP's control signal lines as IO...
From page 16 of the manual in your first post: "Output1 and Output2 are connected to Pin 17, 1 of the DB25 connector respectively,". These pins are "/SELECT" and "/STROBE" signals, and are really control outputs for the DB25 parallel port.
You can check this with a volt meter...
For those outputs, all the MX unit supplies is opto-isolation and a 1k Ohm resistor per Figures 16 on page 17. So you will have to supply "+V" and a Stepper Driver per the "Expanding the 5th axis" notes on Figure 21 bottom left corner box.
You will likely still need an external temperature controller for the hot end...
Again, I didn't read the whole MX document...
Good luck.
You will also need an output to tell the temp controller when to start controlling the temp, and likely an input to tell M3 the hotend is up to temp...
A few links you are likely aware of...
http://www.instructables.com/id/CNC-to-3D-Printer-Conversion/http://www.jonshobbies.com/convert-a-cnc-router-to-a-3d-printer.html