>>1319436First priority for your bike is cheap to maintain. Pic related, my courier bike. If your Sora Shifter brakes, you'll be out a day's wages to replace it.
Everything you carry needs to be water-proof. Don't spend much on special clothing, a simple non-breathable water-proof jacket or cape is enough, everything gore-tex will soak through at some point anyway.
Basically, go cheap on everything, or you'll lose money while working your ass off.
Most important thing to maximize income is to be friendly and talkative, to your dispatcher and your customers. If you ride fast, but the front desk girl lets you wait cause she doesn't like you, you're slow.