Great points here. Also remember that if talk monthly payments with you then they will tell you they can get it way down, and they can, but, you'll face a rather large balloon payment at the end of the loan...this is a big NO NO and can really hurt.
What I would suggest doing is look at your budget and determine what kind of monthly payment you can afford. Lets say hypothetically 300 a month....thats puts your loan at about 16K for a 60month at 4.5%. So lets say your walk out price on the Stang was around 27000 (by walk out i'm including TTL).....this would leave you to come up with about 10K in tradein and down payment to get your 300 a month payment.
If you do a full finance then obviously the monthly is gonna go up, probably around 500 i would imagine.
The best thing to do is figure out what monthly you can afford, then goto one of the auto loan calculators on the net that allow you to enter your monthly and pay term & percent and it will tell you what the finance would be...kinda reverse engineering your monthly.
Lastly, you probably need to identify what the difference is going to be on your insurance as well and figure that into your budget. Sounds like a lot to do but in the end it will give you the best idea.
Also, if you use excel there are excel templates on the microsoft templates site that you can use to see your monthly payments and it will also create the amortization sheet for you.
Good luck.