So....I'm not sure where to start. There are a lot of "life questions" that actually come into play. First off, I would agree with rsk that understanding income to debt ratio is an important factor. That said, I don't know if you are putting yourself through school, student loans other debt etc., but what I would say, starting off as debt free as you possibly can be if not totally debt free is a great way to embark on life's adventure.
Other factors that you would want to consider would be the cost of insurance and maintenance and just one person's perspective, when you land your job, max out your 401K and IRA...these should not be considered part of your disposable income. You should also sock away a percentage of your salary into savings and start to also build up some funds to start investing.
Based on a 50K salary, before 401K and IRA contributions, I believe your federal effective tax rate may be somewhere in the mid teen percentage so your take home on an annual basis is in the mid 3K per month range (I could be totally wrong...I'm not a tax guy hence why I have a tax guy
). That doesn't include state taxes (not sure where you live). Now you take out housing, food, clothing, insurance ( pretax medical, dental, vision), utilities, entertainment.....
Let's say a $60,000 pre owned GT...20% down brings the cost of the vehicle to 48,000. Tax, title and all the doc fees - let's say brings the out the door cost up to $51,800. Just throwing in a 3% interest rate for kicks and the monthly payments for 48 months would be $1,146.
There are so many factors here that on top of income to debt ratio, it is also about what type of life style you want, meaning, what do you do with your disposable income.
I believe there are some forum members that are in their 20's so maybe they can provide some further insight.