Are you comfortable with owning a car and maintaining it to reliably fulfill the requirements of the research project? I could keep a beater running for 18 months but might need a lot of help doing the research. Maybe you’re up to both. If not the rental from the university leaves you free to take care of your primary responsibility while leaving the school responsible for keeping you road ready. I would advise checking on the insurance and posting back. A 3 year lease would likely cost more than the rental and a truly reliable car would likely cost well over $10,000 and might require a new transmission($2,500) in 6 months and sell for $2,000 in 18 months.