I would take a non-selective herbicide (kills anything it touches, so be careful) to the lawn. Once everything is dead, till it up until it totally decomposes. Rent a bobcat and get rid of any extra dirt you may or may not want, or re-grade. You'll have to wait a while to seed after applying the non-selective herbicide (6 weeks probably). By then, a few new weeds will appear, so take a rototiller to it again right before you seed. I would recommend hyrdoseeding, as it is the fastest, and applies seed, mulch, and fertilizer all at once. Slit seeding also works wonders. Be sure and apply a starter fertilizer at the same time you seed.