IMO, you should easily be not only maxing your 401k ($18,000) and your IRA ($5,500), but you should also be putting $5-$10K in a taxable brokerage account (or saving for a down payment on investment real estate). I see no reason why your car expenses + "fun money" budget should be more than $9K/year (or less).
Also, If you want to sell the car in favor of a $5K or cheaper one, have at it! FYI, there are plenty of fun older sports cars (or whatever your preferred style of "car [you] always wanted before [you] have kids" is) to be had at that price.
You may have forgot taxes in there...
After car payment, with those savings numbers I'd get less than 4K for the year, including gas, car insurance, girlfriend, etc.
I didn't forget taxes; I ignored them because I assumed they were negligible. Let's do the math to find out:
$42,000 gross income - $18,000 401k - $5,500 IRA = $18,500 AGI. $18,500 - $6,300 standard deduction - $4,000 for 1 exemption = $8,200 taxable income. Tax on $8,200, filing single, is $823. Saver's Credit would be $400, so net Federal income tax would be $423 -- yep, that's negligible!
State income tax is usually (or always?) less than Federal tax, so it would also be negligible.
So the OP's budget would be $42,000 - $18,000 401k - $5,500 IRA - $423 Federal tax - $250*12 car payment - "$5-10K" taxable investments =
$5,077 - $10,077.
Even $4000 (let alone $5,077 or $10,077!) seems like
plenty of money to cover "gas, car insurance, girlfriend" to me. Note that per the OP, there is no significant "etc."