Yes, by today's standards, the engine is considered weak. But, many people including myself consider the 7.3L to be virtually bulletproof and highly desirable.
Years ago when I was selling Fords, by BIL got the a 7.3 right before they switched to the 6.0L. He's not known for taking care of his stuff, he's beat the truck up and does minimal upkeep on it...but for as much as he has turned a really nice truck into a POS, the engine still runs great and he's had zero problems with it.
Since you've owned a couple and know their weaknesses, if it's in good shape and appears to be taken care of, then go for it. Just make sure to use this stuff.
https://www.amazon.com/Motorcraft-Diesel-engine-coolant-additive/dp/B0065SZU8I