Need help with Cat Converter

Hi,
I am new to this forum and I desperatly need some help.
I recently purchased a 98 740il with 109k. After a single week of driving this car my joy was cut short when the "Check engine" light came on. I had the guy over Autozone pull a bunch of codes which has to do with the O2 sensor "Bank 2 sensor 1", Cam Shaft sensor and the catalyst converter. I saw this thread and I could not beleive the price that the Maganflow costs. Comparing to the $1900 AM/ $3200 OEM quote that a local mechanic gave me I think it is blessing to know that some of you paid that much for it. So to get to the point can someone point me to where I can get both the right and left Cat converters from Magnaflow and which one do I get? Is it 94006, 53005, 54005 or 94006? Also does anyone know which sensor is "Bank 2 sensor 1" before Cat converter or after. Do I need to replace all of them at the same time or wait until they die one by one. Any help would be greatly appreciated. Thanks