Potentiostat circuit components
R6: value is not very influential, as the signal from the sensor WE terminal is expected to produce a particular current, with the op amp circuit acting to translate that current to a voltage. The op amp will adjust its output to a voltage that causes the same current to flow in R4, with the feedback action setting the op amp neg input to 0V. So output voltage will be WE current x R4 value (10k).  So actually no R6 is needed, but it's a good idea to have some resistance there which would limit current if the sensor wire for some reason contacted an untoward voltage.  1k or 10k should be fine.

The P-channel JFET is there only to create a short circuit across sensor RE and WE terminals when the circuit is powered off, as explained in the application note.  When powered on, the +V applied to the JFET input should switch it off, making it effectively disappear from the circuit.  So we want a JFET which switches on adequately with VGS = 0, and switches off adequately with VGS=12. 

The JFET off characteristics are the primary concern, as leakage between D and S, or G and S would add to or subtract from the sensor current. The degree of concern depends on the current expected from the sensor, which presumably varies by application. The article discusses a 500 nA signal, in which case leakages of 5 nA constitute 1%, and 50 nA is 10%.

FWIW, the P-channel JFET, being normally on, is not nearly as popular as N-channel, so there is some lack of variety readily available.  Looking at Jameco, the 2N5460 looks like it might be good enough for &lt; $1.  However, given the concern in the article for low-leakage ("bias current") op amps, the same concern may well apply to the JFET, so you might look for a lower-leakage JFET -- with the 2N5115 popping up as one that has significantly lower leakages (VGS-OFF 3-6V at 1nA IDS, IGSS &lt; 500pA.  No doubt there are others.

Hope that helps.