is a handwritten agreement that is signed an enforceable and valid contract and could it be used as evidence in court

The fact that an agreement is handwritten (versus typed) likely has no bearing on whether the agreement is enforceable. Contract law was developed quite a long time ago when equipment to type out the contract was not commonly available. Even today, many people do not have ready access to typewriters, printers, or other equipment.

The law of your state may require that certain contracts, such as for the purchase and sale of real estate, must contain specific information and follow certain procedures.

You likely should review your facts and options with an attorney.

In Florida it does not matter whether a contract is handwritten or typed. Remember, an oral contract must be for less than $500, must be performable in less than one year and must not be for real-estate. Everything else must be a written contract. You could even write the contract on a napkin, both parties can sign it, and enter the napkin into court as evidence of a contract.

Generally, the answer is yes. In fact, in many states the document only needs to be signed by the person that you want to use it against and not by you too. Contract Law is pretty much the same everywhere but you should check with your local Bar Association for a Contract Law attorney who can give you specific answers for your state law. Most states allow oral contracts (like a handshake agreement) to be enforceable too as long as it is for a relatively small amount of money, but proving who said what and what was agreed to is pretty much a "swearing contest" at that point. The common advice would be that if what you are agreeing to involves any substantial amount of money or value, like $100 or more, then you should always write out a simple handwritten agreement for everything you are agreeing to. That way there is no dispute about it later. Just remember that if anything is ambiguous or fuzzy about what is written down, the law generally will interpret the fuzziness against the person who wrote it that way; so, make your handwritten agreement simple and clear and concise and have everyone involved sign and date their signature too.