Though some may consider my methods a bit unorthodox, I'm a get it done with what you have kind of guy when I feel it is feasible. Even if they are old, those 5/05's are some nice processors. There is no need to upgrade because you want to control a valve.
Take your control cabinet, put a 3" KO in the side and set yourself a small panel with a 2.5-3" nipple. In that panel add an old 4 slot rack you have lying around and pull you some power from your panel off one of the buses into that nipple. Run you a ribbon cable over, insert your analog out card (and the analog in card if you need one). Quick question here: Are you SURE there isn't a single spare analog output you can use instead of this? Is the RTD already in the program? Anywho, if you need some extra rack space this solves your problem providing you have enough room to mount a small panel. No need for point IO. You are trying to do this cost-effectively, not re-engineer the whole system that works just fine.
Now, I would need specifics on that transducer you have, but it likely takes a 4-20mA and outputs it to something like a 0-1A valve or such. Now you will need to do some fiddling to get it right, but you can just use an SCP instruction. Couple key questions to help you with this. What is the hottest the water from the radiator ever gets? What is the lowest temperature the water from the radiator ever gets? You can then scale that valve to open and close as needed to cool the water going into the drain.
You would be using a PID if you were trying to keep the water temperature constant, which you are not. You are just trying to keep it below a set point. This system would be a bit reactive, but I feel it would accomplish what you are trying to do, be much simpler than you think you need this to be and would be a LOT cheaper, especially if you have a small PSU and 4-slot rack laying around.
If you think this system would be expanded in the future, go ahead and wire the analog IO up to terminal blocks, put yourself a hot bus/24VDC bus depending on what kind of descrete IO you use, and a neutral/0VDC bus, once again depending on what kind of IO you use. Then as you add a card you can add/label the terminal blocks for future expansion.
Anywho, that is my two cents with the information you have provided. Good luck!
EDIT: Just checked your valve specs. You don't even need feedback from this. It is just something else to go wrong. Your RTD IS your feedback.