The other two sign the document as well.

Ensure to read the lender documents.

Definitely get in writing the deposit amounts from the landlord and I would request the tenants individually sign the document indicating they agree on the amounts.I say this because I bought a multi-unit building a couple years ago.

It makes your job simple, you don't need to negotiate, all you do is file, have the correct documents, appear, and the judge makes the decision.

Difference is the documents are usually only 2 pages long instead of 5,000.

Either your cousin has not quit claimed and it is in his name but you have documents indicating that you are a 25% owner and have been for years.

I agree if you cant provide supporting docs you don't deserve a high price offer and the property could go higher if he had documents but without them the highest I could honestly see paying is $170k

Than being an assignment contract, I would just get an assignment agreement signed by a Buyer to purchase contract for fee, than take these documents to the title company to process the transaction?

Whomever filled out the document is your 1st point of contact. 

I've found TurboTax to work pretty well for me, as I've always thought the hardest part was getting all of your receipts and documentation together.