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Your going to have to compromise somewhere. First issue is you at least $2k upside down, and that's considering they don't figure out the tranny has issues. Cut that value if half if they do and your looking at $3k or better in the hole. So you are either going to have to pay thise difference out of pocket or roll it into the new loan. Getting $30k+ car, rolling in $2k+ and keeping payments under $450 just isn't going to happen. Leasing in general is not a sound financial decision, especially if you have to put money down. I think you are going to have to find a cheaper car to keep your payments where you want, especially considering you are upside down on your current vehicle.
Some other questions to ask in regards to lease:
How many miles do you average a year? Guess high. You don't want to pay expensive overage or have to leave your car parked because your running out of miles,
How well do you take car of your car? Wear and tear damages at lease end can be very costly. That buggy scratch or door ding at the grocery store, the spilled coffee, etc. If you have kids or transport pets tread very carefully.
If the bumper to bumper warranty at least as long as the lease? You do not want to have to put expensive repairs into a car you don't even own.
Just some things to think about, I'd buy a cheaper car you buy to own.