He basically want us to hand him a cashiers check with one hand and he will in turn at the same time give us a notarized deed.

You have a "notarized memorandum" of what?

The underwriter on the deal had me send in a Notarize letter giving her permission to use the money in our joint account .The reason for the letter was because we were not married at the time,  She was using the money in that account for the 3.5% down payment and for closing cost.    

Police dept had me sign a notarized letter authorizing them to pick up loiterers in the MHP.

Does the seller have to have a notarized letter explaining the delay?

He does have a notarized DT in his possession that he has yet to record.

While they may be evidence of a lien release, its possible they may dispute them later, also most county offices require a document to be notarized to be filed in their land records, and finally if you go to court a notarized document is self authenticating, meaning you do not need foundation witnesses to establish its authenticity.

Or just have a notarized contract and make payments?

If the seller will not do the work, you will need to sign a notarized document at closing promising to do the work.  

So make sure you have every 'I' dotted and 'T' crossed (if you are the listing agent that means that you need a drivers license, copy of the listing agreement and perhaps a notarized letter from the seller).I'm surprised that with a complete rehab needed, you didn't have the buyer be responsible for the CO.