Some other plasma cutters have 1:50 voltage dividers too .... 1:50 is the "common" voltage divider setting..... although the optional extra machine interface board for the PMX 65,85,105 (and I think 125 too) also comes with dip switch selectable 20:1; 21.1:1; 30:1; 40:1 and obviously 50:1 divided arc signal. pdf page 68/248 here (and a few pages before shows the optional machine interface & voltage divider).
http://www.air-works.info/Hypertherm%20Powermax%2065%20Service%20Manual%20807120r1%20EN.pdf
This is actually a better manual:
https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0ahUKEwjiusyU-YDQAhXnJ8AKHYh3AP8QFggtMAM&url=https%3A%2F%2Fwww.hypertherm.com%2FXnet%2Flibrary%2Flibrary.jsp%3Ffile%3DHYP103957&usg=AFQjCNE3T2d_5tkjgBs6m4eqKSqyzAVGTw&sig2=7W4JbjTPhLOq5SJJkP7HOg&cad=rja
If you have a look at pdf page 4/16, it covers both the powermax 45, and 65-125 interfaces, and provides guidance in the highlighted box about isolation (see below too)
But if you you at the Proma, Neuron, MiniTHC they all have 50:1 inputs or raw arc voltage inputs...... raw arc voltage does open up your market because I know a lot of people have bought cheaper chinese units (hand plasma with HF arc transfer) and as much as I do not like them.... they are a fact of life and a market which can and is being tapped into by the proma for example, being a lower cost THC, and it catches the entry part of the market before customers are like me and want something more
The isolation question...... I would say that the 1:50 voltage dividers are NOT isolated..... because of the above referenced second link to the manual for the interface, and also if you study the last page of the service manual for the powermax 65/85 it shows the voltage divider circuit (although crudely).... as being non-isolated (bottom Right hand side of the schematic 1:20 voltage divider output is standard, unless you buy the interface board)
https://www.hypertherm.com/Xnet/library/library.jsp?file=HYP109602&format=pdf
However, as this point (as I've looked at building my own THC quite a bit....[not for sale] ) I did a bit of research as to what the higher end / older hypertherm plasma cutters did, and I found that they tend to use the AD202 analogue devices Isolated opamp....
How did I find it.... ebay search.... "hypertherm + divider"
http://www.ebay.co.uk/itm/HYPERTHERM-VOLTAGE-DIVIDER-ISOLATED-for-Arc-Voltage-Height-Control-Pt-No-041274-/131184598033?hash=item1e8b361c11:g:K0MAAOxyThVTaXww
The AD202 is not cheap, and the manual for the AD202 is here http://www.analog.com/media/en/technical-documentation/data-sheets/AD202_204.pdf. The negatives are the cost, +/-15v power supply and the manual has some good examples of circuits and the use of an AD711.
When the AD202 came out I don't expect the IL300 was out.... and may provide a more cost effective isolation solution
http://www.vishay.com/docs/83708/appn50.pdf
I suspect and I'm not really sure about it .... but I wonder how many THC's really include for isolation on the 1:50 voltage divided input. Given there is no secondary side power supply supplied by the plasma cutter via the machine interface...... hence really the THC would require an isolated DC/DC power supply too
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With regards to your
0.14655... Volts per bit. (Volts of the plasma arc)
the thing to bear in mind, on 12 bit resolution (4048) = 593 volts.... great.... but most of the time you're not interested in anything over ~165v or anything below ~75v.... hence my suggestion for two channel (high and low resolution) given we've got two AI's .... because if you offset and amplify then your resolution (+noise unfortunately) will improve.
at 0.14655V/bit, that only gives you 6 bits per volt (which is ~0.5 to 0.381mm of Z axis height) .....
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If you're thinking about plasma interface boards , then a suggestion would be to think about doing what was done with the MiniTHC.... have a direct machine interface port.... given there are now a few plasma cutters that are sharing the same port
http://www.ebay.co.uk/itm/MiniTHC-torch-height-controller-THC-/322277500765?hash=item4b093c675d:g:EwwAAOSwBahU56lm
Connector here + part numbers
http://www.ebay.co.uk/itm/DIY-CNC-Machine-Interface-Cable-Connector-For-Hypertherm-023206-228350-223048-/252548420484?hash=item3acd0f1384:g:hZ8AAMXQWzNSiYz-
think about maybe a high speed relay interface (reed switch dry contact [thanks to Keith for educating me on that one])
https://4donline.ihs.com/images/VipMasterIC/IC/COTO/COTOS00065/COTOS00065-1.pdf?hkey=EF798316E3902B6ED9A73243A3159BB0