I've seen this thread before and even more curious now because I do not have a heat exchanger. To me, I'd like a heat exchanger because mine like so many others runs hotter than hell, can't touch it hot. However, I have to figure this has been like this for more than 5,000 hours probably more like 10,000 hours.
I just have to imagine a heat exchanger would be beneficial from the engineering point of longevity. That however is disproved by what we all see, do and repeated in so many places. I just know even on the smallest of hydraulic systems outside of small boats, when we get on ships even on 1 gallon sumps I see freaking heat exchangers...
After the very first run with my boat during sea trials, I could barely steer the boat when approaching the dock; the wheel was wicked hard to turn. Discovered that I could have lit a match off the helms they were so hot. We immediately installed a Sen Dure (Ft. Lauderdale) heat exchanger and ever since then the helms and the hydraulic storage tank stay nice & cold and I have had no further issues. I have model 2254-1-7; it has two pencil zincs and other than replacing those, there's no maintenance. You probably want to remove it for cleaning occasionally though.
Install it between your raw water basket strainer and the engine. The internal tubes of the heat exchanger (through which the sea water runs) are very small and your hull-mounted strainer won't filter out stuff that small and you might block some tubes and starve your engine.
Sorry, don't have any numbers as far as cost, but I'd recommend one if your fluid is getting that hot.
That's stock Duffy steering. Something is wrong. Check your oil level and see if the oil is dirty or maybe it is burn't. Or the level could be off to low to hot to high poor circulation. You may also have air in the system which is making the pump work to hard.