The question is pretty much in the title but when putting characters together, there's the potential for 24/ 25 point sets. Can these hold up vs 30 points if characters and if not, where's your line that you won't go below. 28?

I feel like that's going to depend entirely on who the characters are and how the deck is built, but so far I've managed to not go under 27 without feeling underpowered.

Much lower than that and you may as well bump one up to an elite to fill the pts in.

Play it on Champion Mode, Solo Tusken Raider.

If you've maxed out all of the character's you've chosen to play and do not have enough points left over to bring in another character, you've done the best you can do with your character points. I can't give you a number other than 23, as you clearly have enough points left over to bring in another character.

Your deck is going to have more to do with your success than the characters you've chosen will.

I do want to try out a 23 point hero deck (the cheapest hero is 8 points) and a 24 point villain deck (the cheapest villain is 7 points) against a few 30 point decks and see how they hold up. If they can't win consistently, then I'll have to try 25 point decks and so on until it is about 50-50.

That depends, are we presuming you have access to all the cards and dice you need or looking at casual play where you sometimes won't have a second dice to give a character or the card for a cheap character to add?

In the first case I'd be pretty skeptical at the point when you can afford a new character if you remove the second die on one of you current characters (depending on the second die of course). So that's around 26 points.

In the second case it's not really possible to say since the quality of cards in the deck can vary so much.