IMHO, the absolute maximum that you could in any way justify for basic transportation would be somewhere around $10k. You can get a pretty nice, 4 year old car that's in very good condition with about 50k miles. I'm partial to the Mazda3 hatchback myself because they generally are a little more fun to drive than the equivalent Honda or Toyota and have poor resale value because people aren't as familiar with them. Have a pre-purchase inspection done, and you should be good to go.
However, $10k should really be an upper limit for a mustachian. Sure, if you have $500k saved up and are coasting in to retirement then spending $10k on a car is fine, but at that level you're spending more for an illusion of reliability. Starting somewhere in the late 90s to 2000s, quality really improved dramatically across the board. If you buy something with under 100,000 miles on it and it's less than 7-10 years old you should be fine as long as a good mechanic checks it out. In addition to helping you avoid buying a car that has issues that aren't obvious, a good pre-purchase inspection will give you a list of things that you can use to knock down the price. I don't remember what I paid for my most recent PPI (maybe $100-150?), but it came back excellent but there were a few minor things they found that I didn't - one key fob didn't work, the battery was going to need to be replaced soon, the wiper blades were bad, and one or two other very small things. I was able to take the report to the place I was buying the car and get them down an extra $500.