I know this is an older thread, but I just stumbled on it.
I am an alarm installer, specifically DSC products. If you are looking to use the individual zones from your older 5010 system, there are a couple things you need to confirm.
First off, looking at only wired zones, contacts and motion detectors are either 'normally closed' (most common), normally open (less common), and either NO or NC with end of line resistor (least likely with residential).
Assuming that all contacts are Normally Closed (NC), you could use the GPIO to check if the zone or pair of wires are closed (normal) or open (alarm state).
Most motions require 12VDC to operate and will show as being open if they are powered down.
Once you have assigned the GPIO pins to the zones, a simple monitoring of these pins and notification if change is all that appears to be needed.
On the wiring of the existing 5010 board, the zones are fairly easy to identify, with one wire of the pair going to a "C" or common terminal. Generally the wires of a 4 wire quad wire use yellow and green for zones, black and red for power (if needed).
I hope this makes sense. Its pretty simple but could look scary depending on how messy the tech was that did the installation.
There can also be expander boards if your system needed more than 8 zones. These boards also work in 8 zone configurations (on 5010) and are about the size of a RPI zero.