If the ultimate goal of leasing is to own the car after the lease ends then it is a very good strategy. You scores are a bit low for leasing but depending on what kind of car you are getting, it is doable.
No one here can tell what interest rate you will be getting as a lot of factors go into determing that. What kind of car are you thinking of getting?. Also, getting someone to co-sign just gives you a better chance of getting the car if their credit file is better and does not change anything. They will be joint on the account and will report to both credit files.