No problem with that.
A lawyer is not required by law, of course. It's your call and it depends on your confidence about properly filing paperworks.
I suppose the future wife is in Mexico, and has no pending issues with USCIS, right?
Best and correct way to do it is sponsoring her for a fiancee visa (petition I-129F). She will process it at the nearest US Consulate in Mexico.
Once she obtains the visa, she can enter the US and must marry you within 90 days.
Then she could file for Adjustment of Status for her green card.