1. "Just bought a used V70 XC 1998. Noticed that the "cabin" creaks when torquing the body while driving over dips/etc. at an angle, like into the driveway. What might cause this? A problem or normal?"

Unfortunately yes. Volvo has a service bulletin covering noise reduction. It is
Sec 8, Group 88, No 003 (15 pages).

2. "Also, when backing off the accelerator, the transmission will upshift with a clunk noise as if gears were meshing roughly. A problem or normal?"

Normal if the transmission fluid is old or has never been changed. See Bay 13's
instructions for flushing the trans fluid.

3. "Right rear suspension groans when cornering or driving over dips slowly (when I can hear it). Cause?"

Could be a worn Nivomat shock or the "helper spring" bump-stop retaining clips.

4. Last, do all the 2.4 liter engines seem to "growl" on acceleration or are they supposed to be smooth and silent? Timing belt problem? Valve situation?

Yes the intake on these engines is very loud on WOT. Make sure the airbox clips
are secured and all intake hoses are ok. And while your at it check the upper
rear engine mount which when worn can cause vibrations and noise.