Doing homework before you go ahead is wise. But remember no 100% guarantee will be there for you whether you do it by yourself or by a lawyer.
The choice mostly depends on your finacial situation and your background (strong or weak). The best way is to call the lawyer and you will get the impression about the lawyer from what he talks.
If you do not want to pay lawyer; your background is strong; and your English is not so bad (if you have written some English paper, your English is good enough for this purpose), I would strongly suggest you do it by yourself with the help of good NIW package.
Make up your own mind. Good luck.