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I was kind of in the impression both expensive devices already exist.

If the boiler water can be in the range of 120F to 180F you want the automatic limiting action of the thermostatic mixing valve.

Cancel each other out ? They work in concert.

" I should have also noted this is just one zone in a larger system. "

Something in the rest of the system may need the 180F. So the thermostatic mixing valve limits the water temperature to the appropriate temperature for that section of the system.

If your old boiler is way oversized and you experienced short cycling in the coldest days of the winter, then the replacement boiler is too big. If the old boiler didn't properly heat the home then it might be too small. If Goldilocks selected the old boiler and it was "just right", then that replacement boiler will probably work fine.

What I am saying is: to select a replacement boiler you should have a professional measure your home for a heat loss calculation. Many older homes had Coal Boilers that were converted to Oil and even to Gas. When those old conversions were replaced, the plumber may have selected the replacement based on the coal boiler size. Many of those coal boilers were oversized by as much as double or more. If your home's heat loss is only 60,000 BTUh then that replacement boiler is way too big.

But if you are asking “will the new boiler do the same job as the old one”… it is very close to the same size so it will do the job in the same way as the older boiler did, short cycling, wasting fuel and costing you more than you need to spend for comfort.  And who wants to spend more on fuel oil at today's prices?

Think about the last time it was the coldest day of the year and the old boiler was operating to keep you comfortable. Did the oil burner shut off for 10 minutes or more a couple of times during the hour? If it did then it is too big.

a boiler that can produce 138,000 NET BTUh installed in a home that needs 60,000 BTUh on the coldest day will get hot so fast that the water temperature will reach the high limit setting and the burner will shut off for some time. When the water temperature drops below the high limit setting the burner will come back on. If the burner operates 10 minutes on and ten minutes off for several hours at a time, then the 138,000 BTU capacity is actually only 69,000 BTUh. It only operates for 30 minutes every hour so it only needs to be a 69,000 BTU capacity heater.  I hope this helps you to decide on the proper course forward.

Mr. Ed