as you have a voltage output on your plc set the valve to this (better would be a mA signal ) but use what you have it will work.
the valve should also be in voltage control, As the feedback has the same configuration it is impossible to use the feedback (not needed anyway as the valve will not be stuck anyway).
For character i first would use linear and if valve tends to regulate in lower or higher 20% ( below 20% or above 80% change over to exp.) this way you have better control of the temp.
Now use two temp sensors one in your tank and one on the outgoing line this will gaurantee you never go out of bounds and you can control your tank accurate.
for a good PID or control look at www.oscat.de
if you would like it fuzzy (expert system ) give me mail.