He is right. Cash flow is everything, it keeps you going month to month, it pays your bills, it keeps money moving in your bank accounts. Don't throw a large sum of money out at one item, it ill drain your account and make you vunerable in the long run. You need to figure out if you can afford a monthly payment of $125 - $150 a month right now or what would it take for you to afford that payment. Usually it means picking up a couple more clients.
But whatever you do don't buy a bigger or better mower unless you absolutely need it. If you don't have the income to support the payment then you don't need it.