I haven't done it on your specific vehicle, but the O2 sensor replacement should be the simplest of all those tasks you listed. Sometimes it takes a special socket though. The dealer will have one in his tool box. If you buy one, it may cost more than having the dealer simply do the job.
Replacing the serpentine is typically simple as well, you just need to figure out the geometry of leverage to be applied to the belt tensioner. Rear wheel bearing failures are rare, and changing them is much more difficult than front bearings. That's a job for a pro. Can't address the vacuum seal without more info. Blower motor replacement same thing. Degree of difficulty is commensurate with how hard it is to get at it.
In all, I think $2500 sounds steep. You might want to get another appraisal.
//greg//