Ok, here is a long story short.
My grandmother bought into a piece of commercial property back in the mid 70's. It is an industrial building used by a plastic factory in LA, CA. The agreement was all done as a handshake with a friend. Let's call this friend Bob. There was a large monetary exchange. She then owned that property for 20 years and Bob paid her fair share of the rents. When she passed away she gave it to her kids (my mom and aunts). Her kids have been getting paid about $10k/yr each for the last 20 years. Everyone was happy with this. 

The Twist 
Bob just died and left his kids the property. Bob's kids had no idea about my grandmother's family. They stopped paying and thought someone was potentially stealing the money from their aging father. There is no written contract to fall back on and any bank statements of the initial purchase are long gone. 

Where we are now
After sorting it all out over the last 6 months, Bob's kids agreed to start paying again and to formalize the partnership. We need to get a good agreement set up. Lawyers of course are involved. Are there any special things we should put into the contract that we or the lawyers might not think of? 

Selling the property
My mom's siblings want to sell out and get rid of this deal. How do they do that? This property has given $10k/yr to 4 people for the last 20 years. Is a 5% cap rate reasonable? That would mean each person's share is worth $200k each. Is there any other advice on valuing the property? 

Thanks!