you can finish every single wizardry just fine without class changing. oh, and, wizardry 4 doesn't even allow class changing! you play only as protagonist character who is pre-generated as a mage.
"class changing is the opiate of the masses!" - Maribel (dragon quest 7)
as for what karma does... all I know is you should re-roll until you get 10+ on it. it affects various things and basically works like LUCK stat for shit under-the-hood that, most of the time, you will never know what is being affected.
now... as for the seeming issues, "complaints", and other misc. observations you seem to be posting about regarding the gameplay mechanics of wizardry 7 (and/or wizardry 8)?
You are completely correct in quickly ascertaining that shit like regenerating mana/health and the insane amount of trash mobs that populate "trash areas" (a first in wizardry series), the mind-bogglingly retarded implementation of bad puzzle designs (and bad puzzles, period) in 7/8 and the stupid, stupid, STUPID and disposable NPCs and meaningless errands you that comprise the bulk of w7's "gameplay", and lastly the hyper-retarded and badly implemented "feature" of competing parties (they glitch/break the game and cheat you out of items, instead of actually enhancing the game world/reactivity)...
YOU'RE IN LUCK! I have a simple solution:
- play wizardry 3, 4 and 5 ports for ps1 instead! amazing orchestral soundtrack, polygonal and textured dungeons, option to change to classic wireframe graphics and that includes seperate option to change enemy sprites to original ones as well, instead of the updated ones.
- all wiz ports on ps1 are content-identical to the dos versions, thus completely eliminating the need to play those.
EDIT: wiz6 is also a nice balance between the remnants of (true) Wizardry gameplay from titles 1-5 combined with the beginning of the end of the western-side of the series as Bradley begins introducing his dumbed down mechanics.
wiz 6 is much better than 7 or 8, and wiz 1-5 are better than wiz 6; however, taken on its own merits, wiz6 makes a good case for being the best balance, the best mix of old wiz and "new wiz".
tl;dr: play wiz 6.