If I wished to change the stats that affect a caster's spell dc and caster spell stats (say to make favored soul CHA or WIS only instead of both) how would go about doing so? Would it just be a matter of editing the row in classes.2da, or would I have to do something else as well?

It's been a long time since I got into it, but there may be specific references in the script of the spell itself. It may depend on the spell. But I could definitely be wrong.

That would be one thing you would have to do. Try searching the term spell hooking in various forums, like bioware's.
http://forum.bioware.com/forum/157-builders-scripting/

You only need to override the classes.2da. I can confirm that the columns you want to edit are PrimaryAbil an SpellAbil. Just make sure to use a 2da editor like TlkEdit. We changed Spirit Shamans'(now WIS/WIS) and Favored Souls'(now CHA/CHA) spellcasting stats at the Realms of Trinity PW less than a year ago.