There are also strong emotional ties as her recently deceased husband was the one who built it. 

If you lowball them with the offer, present to them an explanation why the offer is low, it could be that they are personally attached to this property and they feel emotionally that its worth more.  

Don't get emotional about it.

@Kyle BiggerDidn't read all of the responses but you can switch to commercial where there is little emotion involved.

The letter to the sellers is more about emotion, and like you said, standing out in competition with an all cash offer.

-last make sure you are conservative on your days on market and the holding costs after the construction is complete.When we are running our numbers we won't do a project if it doesn't have a 20% ROI, we don't look much at the dollars as that can push you to make an emotional decision that could cost youGood luck!

It helps take some of the emotion out of it too so you don't worry or wonder what the right thing to do is.

(It also holds emotional value, being that it was my mom's car before she succumbed to cancer 5 years ago.) 

Let go of your emotional attachment to the property.

My wife and sister-in-law will be going through a lot of emotional turmoil at the time this all takes place, and so I'm preparing to be the one who handles the business side of things and I'm trying to learn as much as I can ahead of time so we can not only handle their dad's legacy responsibly, but also grow it over time.Thanks for your help!