Statement of purpose: how important is correctness of grammar and punctuation?
If the written question above is a genuine reflection of your English spelling and grammar abilities, then I strongly suggest you seek professional services to assist if you are in any way concerned about your abilities having an adverse impact on your application.

This application is clearly a very important step in determining your future, and if it is so important and you have concerns, it is not worth leaving it to chance. If you have no suitable friends, use a professional service. Plenty of other native-English speaking applicants who are competing for spaces with you will have done so. 

I can't say for certain how a selection committee will factor your English as a second language. It is likely to depend on the institution and the course you are applying for.