This "PC Data Acquisition Unit" would be a simpler solution.
.... but you miss out on the hacking part
It gives you:
Eight Digital Outputs. Open collector, 500mA, 33V max.
Sixteen Digital Inputs. 20V max. Protection 1K in series, 5.1V zener to ground.
Eleven Analog Inputs. 0-5V, 10 bit (5mV/step.)
One Analog Output. 0.2.5V or 0-10V. 8 bit (20mV/step.)
Small plastic box (140mm x 110mm x 35mm) with screen printed front & rear panels supplied.
Connected to the PC by a 25 pin straight through cable.
Software utilities & programming examples supplied.
Seven pages of documentation plus schematic.
Just use the Analog Input and a simple resistor voltage divider and you can measure any voltage you want.
(CPS93 -- $64.95)