Ok then it's an easy choice - I will try the 5th edition for this campaign. If it doesn't feal right for us we can always change afterwards.

They have experience in 3.5 but it's a good idea to start with lvl 3 so they can start learning how their new characters are working but aren't that weak as a lvl 1 char. That's maybe better than giving them lvl 10+ chars without knowing what the class can really do.

For the kind of evil group I am imagine the villain type of an evil sorcerer with his undead army - the real bad guys. And a big goal they have to achieve like helping an evil but almost forgotten deity to get more power hmmm so they could get their order from this deity. That would let them work together, in most cases I think and still let them fight amongst them to get more power of the deity.
And these evil spell slots are a good idea. With these spells I can also add more stuff that would not be possible mhhh.

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