I was able to the entire job without removing the fan or fan clutch. Getting the fan out of the way would've made the job easier though. This is a potential catastrophic failure item (seizing of idler pulleys) that I was on borrowed time for.
The A/C tensioner & pulley was very simple from below. The alternator tensioner and dual-pulley system required attacking from above and below, but it was still surprisingly/relatively easy. Thankfully I had the Bentley manual to help figure out how to route alternator belt when I reinstalled it!
The irritating loud belt noise/"whistling" at idle has gone away, so the outcome was a success!! There is no doubt the pulleys were ready to be replaced - they had plenty of play and made noise just spinning them by hand!
My radiator and at least the upper hose have been replaced at some time in the past, because I have the newer ('01+) upper hose configuration (and "1Q01" stamped on the hose). I suspect the fan and clutch have been replaced as well. Nevertheless, I plan to replace the water pump and all of that as a Spring '07 project.
Jobman, or others with a '99 540/6, I'll be glad to supply the part number list if you want to do this job. The parts are apparently different if 6-speed and if newer than 9/98. Total cost of parts from AutohausAZ was $234.