Avoiding capital gains on long term investment. Unique situation

[Ben Gerrits]

Hello

I'm working on a deal with a couple who have owned a property since 1989  that I'm looking to acquire.

Here is the quick rundown of what info I've gathered:

Purchase price $89,000

Sale price (pending) $280,000

mortgage owed $190,000

depreciation claimed is unknown 

Property is not claimed as homestead, marriage tax filing under $250,000 combined.

The couple purchased the property with the intentions of fixing it up and renting 1/2.  About 15 years ago they inherited a house from a family member, moved there and the property has been vacant since.  The biggest concern with the seller seems to be avoiding and capital gains payments if possible. As far as I know they have no intention to 1031 into new property, and the home they live is is mortgage free.

What if any, taxes gains do they need to pay in this situation? I understand that more info is needed for specific numbers. I feel like due to the length of ownership and that amount still owed on the mortgage (to be paid back at time of sale) they'll be most likely looking at little to no capital gains.