The tranny is not "weak" but does need to be flushed. Remember, a 5 speed autobox requires 3309 fluid and you will need at least 10 quarts to do a good flush.

Bevel gear flushing is a requirement, at least every 50K miles. Sooner if you travel a lot of slippery roads. What takes the biggest toll is when wheels spin and then hit dry pavement.

Do not be overly concerned with the braking system making a wheezing sound when the engine is off and you pump the brakes. I have heard this sound from 850s and V70s of all ages and does not appear to be a problem. Do replace the vacuum valve if the idle changes when you push on the brake with the engine running.

Vacuum elbows are a pain, but they do need replacing. The PTC valve does need cleaning, they all do. Using synthetic motor oil, 5W30 seems to make that less of a problem, but it is still a problem.

The addition of more electronics was the real problem in 2001. There are 2 or 3 fuse boxes in the car, one in the rear IIRC. If the battery is in the trunk, read the directions on how to replace the battery before you replace it.

In regards to the O2 sensors, I would first check for any leaks around the manifold. Then take it out for a good warm up drive. Once warm, ease the engine rpms to 3K+ and hold it there for a bit. Then do it again at 4K+. Second gear is usually good for that. Then 'blow out the carbon' at 5K+ for very short impulses.
O2 sensors do not last as long when the trips are short and the engine not producing enough heat to burn off coke. If you get a P0nnn that has a bad pre heater text, then you will need to get a new sensor.

For best climate control, leave the AC on, recirc switched off, blower on auto, and vents in auto and temps at desired location. The only time I ever moved the dials in my cars was to engage defrost, but cracking the driver's window 1 inch usually worked better than the blower.

Klaus
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Praying for Cynthia Amendt, heaven bound