Will you need a car after 18 months? What are you going to do then?
How likely are you able to go over the mileage?
Granted paying $300 per month seems somewhat attractive as a fixed cost. However lease are notorious for forcing people into making bad decisions. If your car is over miles, or there is some slight damage (even normal wear and tear), or you customize your car (such as window tint) the dealer can demand extra dollars or force you to purchase the car for more than it is actually worth.
The bottom line is leasing is one of the most expensive ways to own a vehicle, and while you have a great income you have a poor net worth. So yes I would say it is somewhat irresponsible for you to own a vehicle.
If I was in your shoes, I would cut my gym expenses, cut my retirement contributions to the match, and buy another used car. I understand you may have some burnout over your last car, but it is the best mathematical choice.
Having said all that you have a great income and you can absorb a lot of less than efficient decisions. You will probably be okay leasing the car. I would suggest going for a longer term, or cutting something to pay off the student loans earlier. This way there is some cushion between when the lease ends and the student loan ends.
This way, when lease turn in comes, you will have some room in your budget to pay some fees as you won't have your student loan payment (assuming around 1400/month) that you can then pay to the dealer.