Hiring an attorney for the Adjustment of Status stage would cost around $2,000. You can prepare and file all paperwork yourself though. I did that myself, but I also consulted with an immigration attorney through the legal plan at bestimmigrationlawyers.com. The cost was $16 flat monthly fee for their unlimited phone consultations. The attorney, who I consulted, Edwin Lee, offered for me to hire him officially but I declined since I found the consultations to be sufficient and I did not want to pay $2,000 just for him to sign on the application. So far everything is OK with my AOS application and I got the work permits and advance parole documents without any problems. I'm in Texas so you have to check on the website if these immigration consultations are offered in your state. Good luck.
PS: I think one is allowed up to 180 days out of status in the US so your wife-to-be should be OK filing the AOS application.