Generally with all magickal workings, not just spells it should be something that you are deeply connected to, or desire in your life. So you could start with protection workings if you feel that is necessary, if you really need a job you could do prosperity or job workings.
It is all so dependent on your own desires and requirements.
I must agree. This spell is impossible to perform. Magic still follows the rules of physics. It is impossible to change one's DNA and biological functions. No spell can ever force one's body mechanics to function limitlessly. Everything will start to breakdown in time. That is the unfortunate reality.