A statement of purpose/study plan is a personal document and should ALWAYS be written by the applicant himself and not outsourced for a fee. You do not need to be the next Shakespeare but if you are applying to study overseas and lack the confidence of writing one for yourself, then you should introspect whether this is a good use of resources.
There are many examples online and with writing practice one can definitely achieve a professional standard. Its your future, put in the work itself, shed this mentality of getting everything done by an agent