I think you are motoring down the Highway to Hell here. Leases can be tricky enough without packing them with co-signers and a shaky credit rating. You realize of course that up front money on a lease is gone forver, you'll never see any equity back. So I'd use that up front money to pay down the cards, THEN work on a lease deal that doesn't penalize you for bad credit or involve a co-signer. Remember your co-signer is responsible for the ENTIRE DURATION of the lease, so if you default, they are totally screwed on backing up a lease gone bad.
I just don't like the sound of this whole plan for you. Retreat, regroup and re-think would be my advice, offered with the best intentions.