You know, I get what you're talking about. However, I think a bigger consideration would be if you are doing this for yourself or for this band project? If you don't see yourself getting into recording for yourself, then it sounds a little wrong to be buying stuff that you may never use if this band project doesn't pan out, right?
On the other hand, if you're into this so you can learn how to record yourself and possibly upload your playing online so other people can check you out, then I can understand making the investment.
But you've stated that your guitar player already lays his tracks down into his computer, and you'd be recording alone anyway. So this would meant that your guitarist already has the gear you need to record and you would only need a couple of microphones to plug into his audio interface so he can record you.
If you two are going to "exchange" music files where he gives you what he's done on his computer, then yes, you would need your own computer, interface and microphones to lay your tracks down to his music.
If you only have $600 to spend, then that doesn't cover buying yourself a laptop, a nice USB interface and some mics. However, with $600 you can get yourself a couple of really nice mics, and plug into his stuff for recording. So what does he have?
Before you convince yourself that you absolutely need a microphone on everything, you need to learn how to record. And like photographers who buy lenses for their cameras, many learn how to shoot with just one or two to start, and really learning how to maximize what they have. You would be surprised what you can achieve with only two mics - one overhead, and one in front of your kick drum. I would start there. Buy a new pencil condenser mic, like an Audio Technica PRO37, with a boom stand and cable, and then a Shure Beta 52, stand and cable for your bass drum, plug those two mics into your guitarists' rig and see what you get.