Low budget plasma setup, what do I need?

OK, will have to read up on Plasmac.

This might be one way to solve the problem?

bit.ly/2N6SXqX

Maybe. Do you have any analog inputs on your mesa cards ? (7i76e has some).Its coming back to me. I think the sensor I used is an on/off

This was probably it www.jaycar.com.au/ugn3503ua-hall-effect-sensor/p/ZD1902
Find the datasheet. It has a diagram similar to what I used. I machined a plastic ring to protect it.

Can't remember where I got the idea from.

Maybe it all is simplified by setting like this in Plasmac?

Use PlasmaC built in feature to generate ArcOK signal, dealing with plasma cutter guts is not healthy, it does not forgive mistakes.
It should work properly with some fine adjustment, in about 2 to 5 minutes of your time.

I have been looking to get the 'table-frame' more compact in height so here new ideas.

Remember to allow at least 50mm for material warpage below the torch tip and that is barely enough.

Thanks for the tip, will take that in designing the main stand with eventual waterpan and grating or whatever it is called.

I assume that amount of warping occurs on very thin materials?

If you plan on using water table, it will have to be more than 50mm above, then again without water table it should also be more than 50mm. The dust from cutting ( or water ) can easily reach 100mm or more, that is very fine dust that will end up on those rails and soon enough it will make them lock due to tight tolerances.
If at all possible make the table at least 200 to 250mm above cutting surface, make the torch holder so it can be 70 to 100mm above cutting surface. That is easy to do and it will prolong the life of the rails a lot. One carriage is enough to hold the torch and floating head and cables, and a lot more.

Also to add to what Tom said,

The torch airflow will be left on for a period after the torch has been fired. This causes water to be sprayed all over the place from the airflow.

I would allow a min 100mm for the torch to be parked about the water level at the end of a cut.