Is it considered unlawful (haram) to purchase a car through a dealer who buys it and then sells it to me in installments with a guaranteed profit, and is there a permissible (halal) way to do so?

There is no objection to a man buying a car and selling it for a known profit. This is called Murabaha to the one who promises to buy, and it occurs in three stages: a promise to purchase, then the seller buys it so it enters his possession, and then the one who promised to buy purchases it in installments or cash. This type of sale is permissible.