No need to consider another engine.Once the known issues with a 6.0L are fixed, the 6.0L can last for hundreds of thousands of miles.
We recently had an engine rebuilt and then another engine rebuilt and would do it again...just with a different engine builder.
Removing and installing an engine in an E350 is ~40-45 hours of labor.
If the engine is bad, consider having the vehicle towed home or to a quality mechanic that is well versed in 6.0L's in vans.
Bulletproofdiesel has a list of installers - https://www.bulletproofdiesel.com/Articles.asp?ID=373
The closest bulletproof installer to Jackson, WY is Tip Top Transmission
in Salt Lake City.
What about researching how much it will cost to ship the van from Jackson to Sante Fe??
Then it's local and you can approach this onsite?
A mechanic may not be able to tell much until a more intensive investigation.
Is there any maintenance history regarding the engine?
For example - when were the injectors replaced?
1. How attached are you to a SMB E350? Or this SMB??
2. Do you plan to keep this SMB for a long time >>> 5+ years?
***the following questions presume a rebuilt engine is recommended***
3. How much do you want to spend on a rebuilt engine?
4. Would you want more performance or longevity from a rebuilt engine?
5. How much can you spend on a rebuilt engine?