You should get married before the visa expires. Then you have to file for the
I-130 (petition for alien relative which is $355), along with the I-485 (adjustment of status that costs $1,010). With the I-485, you do not need to pay any fee when filing for the I-765(the work authorization). It depends on each state but, usually you get the temporary green card within 3-6 months along with the work permit(if you filed both I-485 and I-765 together). Now to answer your question as whether to hire a lawyer, I hate to say this but it will be better to do so. i know that all the costs adding up is ridiculous but the marriage-based green card is the most complex paper work, and once you make a mistake, you have to do it all over and we all know how much time immigration takes.... And also at the immigration office, the attorney/client line is faster than the individuals line...
You guys will feel the pain now but, I am sure getting those papers right will feel like freedom.
Good luck!