The manifold will have the two holes for the O2 sensors. All the spacer does is screw in the O2 sensor hole and the O2 sensor screws into it. It effectively hangs the sensor out of the gasflow, so it doesn't read faulty.
There are in all honesty, lots of people out there with headers on who haven't had their cars remapped. If you are very very unlucky you'll get detonation from the fueling being too lean, which will lead to damage to the cylinders/pistons/valves. I'm not saying it won't happen as it can, but there are variables such as fuel quality and erroneous sensors that can compound the issue.
K100 is a daughter board that is installed in your ECU. So by looking at your ECU you can't tell if it has a K100 in it or not unless you crack them open, and they are generally sealed. If you buy a used k100, for this reason they generally come as a whole with the ECU, so you'd need to swap it for your original ECU, then have your immobiliser recoded by Honda or someone like TDi-North.
They won't be plug and play, as the map done on it for someone else's mods will be different from the map needed on your car. It will need tuning properly by someone like TDi-North/Eurospec etc.
Did you mean Kpro and K100? Same thing except Kpro is a replacement ECU, and it has a USB port so you can datalog from it and upload maps to it yourself. If you aren't going to be doing that, it's a bit of a waste of the extra money.