re: How bad of an idea is it to buy a new car?

If you live your life by so-called "financial truths" that never take into consideration individual circumstances, you are setting up yourself for problems.

New vs used car is the same thing. As others have written, sometimes, new makes sense and sometimes, used makes sense.

Look at some calculators and analysis of total cost of ownership over a number of years, try to get a low interest rate, etc.