Well, let us get back to the OP and the kind of car he might want to buy- which certainly isn’t a $30000 car.
It can be a good idea to buy a late model used car, if there is still time/miles left on the warrenty. Note that I suggested a 1 year old Toyota or Honda.
Certainly you want to keep your maximum monthly payment in mind when you buy a car. But also- you don’t bargain from there, as Sam Stone said. You bargain from the total price of the car. And, you generally don’t want payments beyond 60 months, nor a lot less than that, either (sometimes you’ll see a dealer advertise a 0% financing deal, but with only 24 month financing. Mosts dudes can’t afford those monthly payments. If you were ready to pay “cash”, then sure, why not?)
Leasing is generally a bad idea. You make a “down payment”, you make monthly payments, and then you own nothing. You could easily end up owing a lot of “milage” charges too. CR agrees with this. However, sometimes dealers have some really incredible lease deal, in which case it can be a good idea.
If you don’t make a significant down payment, at some time in your financing you’ll be “in the bucket” or upside down". This is why you don’t buy a car without that down payment, or money in the bank, or for 7 years of payments.