Shouldn't be to difficult. The diesel would be the easiest as if it has a purely mechanical pump you can cut the lead to the fuel solonoid on the top of the pump at the back and connect a switch to the two ends.

The petrol one would require a switch in the starter solonoid lead, same principle only you would need fairly thick wire unless you used a relay in the circuit.

The diesel I'm sure you could do quite easily yourself, the petrol one you may need help with. A competent self employed auto electrician would be best, although anyone who has a basic knowlwdge of car elelctrics should be able to do it.

How old is old? Older petrol cars should be easy to hook an immobiliser to, just by breaking somewhere in the LT circuit.

More modern cars you need to be careful with. I'm in the process of removing the immobiliser fitted to my Mercedes - because it's only 22 years old it has some quite sensitive electronic ignition components. The way it's currently wired into the coil would probably destroy the ignifition module should the immobiliser operate with the engine running.

However you wire it up you should use a locking key barrel switch.