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It's not completely clear, but it seems you are asking about a circuit in a class 1 div 2 location that will drive another circuit in a class 1 div 1 location.

The point of the IS barrier is to present a Thevenin source.  In other words, the two important criteria are the open circuit voltage and short circuit current.  Since it seems you are connecting to a previously-certified sensor in the C1 Div1 zone, you need to make sure your barrier provides enough voltage/current, but not more than the sensor is rated for.  You have to look at the "entity parameters" of the sensor carefully, and make sure they are not exceeded.  Of course for your own purposes, you also have to make sure that the sensor gets enough voltage/current to operate properly.

Since your circuit is still in a hazardous location, you have to look up what exactly the limits are.  C1 D2 is less stringent than C1 D1, but there are still limits.  Also note that there isn't a single set of limits.  There are different sets of limits, depending on the hazardous gas that could be present.  For example, the limits are tighter for hydrogen and acetylene than for gasoline vapors.

Once you decide what gas you need to be safe against, there are still various interacting limits.  Usually you can derive pessimistic limits that are some maximum inductance and some maximum capacitance, at a voltage level you are guaranteed not to exceed.  It's easier if you can live with these.  If not, you have to look carefully at series resistance and the like, but that can make your certification process longer and more expensive.  This is where you have to read whatever standard you want to claim compliance with very carefully.  Actually, you always have to read the standard carefully, but it's easier if you can stick below the "easy" limits.

One issue you may run into is proving that your 18 V will not exceed 18 V, regardless of certain failures.  You probably can't.  Put another way, the max voltage you use to decide the max capacitance and inductance, isn't your nominal voltage, but something higher you can show won't be exceeded even with some failures.  Then there is also derating of parts.  For example, those zeners will have some rating, but for IS compliance purposes, their clamping voltage will be higher than that.

In the end, a 18 V supply might be looked at as a 24 V supply worst case, for example.  The maximum capacitance and inductance allowed in the circuit will then be a function of the 24 V figure.

It gets complicated.  If you've never done this before, I suggest you get a consultant that has experience in this area to guide you and look over your design this first time.  Otherwise, you'll probably waste more than the consulting fee going back and forth with the certification agency until you get something they are willing to certify, but that still meets your needs.