There was a case a dozen years ago where a guy did an 1031 and then received an unsolicited high offer to sell his new property.  Ruling was that he acquired the new property with intent to sell, not to hold, so he lost his 1031

Yep...and this is the topic of a recent thread on a different list-serve.  I *think* in that one, the high offer to sell actually happened while the properties were in escrow.
@tamurack  - more details, please.

The property was acquired about 18 months ago and has been a rental property.  They are going to exchange for another rental property to be built on acreage that they already own.

18 months should be just fine...

Does the land/acreage price equal the net selling price of the rental?  If not, the construction cost during the 180 time frame *will* qualify but finding an intermediary to handle it may be difficult.

http://www.exeter1031.com/construction_1031_exchange.aspx May make for an introductory reading for what you describe.

I agree with TGB. 

FYI Here is a free one hour CE starting in just a few minutes

 https://www.cpaacademy.org/webinars/a0D4400000QlIZMEA3