I understand how it's going to be a hassle to borrow your moms car. I've been there done that. I would wait until August and buy the car assuming you have the money to do so. This way it is closer to the time you will be starting work. I would also make sure this job your getting is going to be steady and something that is going to last a while. Especially if you are going to be making payments on this car. You are also going to have to worry about gas maintenece insurance etc. The last thing you need is to lose your job and be stuck with something you can't afford.
Definately wait as long as you reasonably can. You want to settle into your job before you buy a car you can't afford if you get fired or have to quit. Just play it safe unless you absolutely cannot.