2000 Volkswagen Passat Emissions warning

I've consistently gotten an emissions workshop warning on this car for two months. According to a read-out from AutoZone, it could be low fuel pressure, a vacuum leak on the engine, or a failed oxygen sensor.

I have two separate warnings on the slip for an oxygen sensor. So, I'd be surprised if I didn't need to replace that, but at the same time, recently, it has been shaking a little while idling (not all the time, but sometimes), and if you're sitting for an extended period, the check engine light that usually just stays on will start blinking. After driving for a bit, the shaking will usually stop. And also, on occasion the last day or two, I've noticed my brakes being less responsive now and then (and I just replaced the brake fluid). What does this most sound like to you? What specific parts does it sound like I need to replace and how difficult are they to replace? Luckily, I have a friend or two that are really good with cars so if I just know what I should do, I should be able to get it done cheaper than taking it to a VW dealer or even to a local shop. Thanks.