hi stu:
on your 02 sensor issue. you might need to replace the front one only. also, check for leaks in the vacuum lines (especially the elbows). volvospeed (bay13) will show you where the most likely spots leaks would develop. also check under the battery tray. there are a couple of vacuum lines that crack over time. you might not even need 02 sensors at all (if previous owner chenged them already). vacuum leaks usually gives you 02 sensor faults.
regards