If I am a buyer of a piece of equipment and I put a down payment does the seller have any attachment to the piece of equipment, and if a contract is signed before the notarized date is the contract void?
It depends on the terms of your purchase agreement.
1) The seller still owns the piece of equipment until you pay for it.
2) Contracts are not void due to informalities. Specifically, the contract does not need to be notarized at all, so it doesn't matter when (or if) it was notarized as long as it was actually signed.