I was basically in the same position as you. I have a part-time job as well going to school full-time.
I had an agreement with my parents that I would pay the down payment ($5k) which I saved up all summer and I would pay half the finance every month as well as gas. They are paying for my insurance. My finance is roughly $350 every month so I guess $175 per month is reasonable. Gas is another big factor but for you, coming from an e46 shouldn't be a big problem.
Personally, I do enjoy a nice ride and that is why I went with my decision. I do not see why not go for it in your circumstance if that is what you value more and if you're able to afford it. If you think about it the other way, the money you will save from not buying the car will be of perhaps greater use.
2007 E90 328i - Titanium Metallic l 19" VMR V710 l Mtech Rear Lip Spoiler l Eibach Sportline l AA N52 Signature Exhaust l MTEC V3 AE l 6000K HID l MTEC 4350K Fogs l MTEC License Plate LED l BlackVue DR380G-HD