tl;dr: I'd advise 38 to 50 for the arc
Longer answer:
Well, back on live, I managed to handle the arc in testing with characters ranging from 25 up to 50. Back in the day, I made sure to finish the arc solo with all hero archetypes, for test purposes (although, I think the Defender had to be a purple IO character to do it)
To be honest, it ramps up to a pretty tough fight at the end. I know some people grabbed teams to finish it. And in his review, Venture mentioned getting trounced in mission 5, but heading back in with a whole bunch of high-level inspirations for some very satisfying revenge.
I worry that the last mission will be too tough for some, but also, the final fight could easily be too much of a cakewalk for a fully IO'd out or heavily Incarnate characters that are around nowadays. Several fans of the arc did tell me they didn't mind a few losses in last mission, as they could get back in, and enjoy overcoming it, and it fit the story. Also, there is a custom character faction of minions, which will probably be just fine, or even on the easy side, near standard difficulties, but if you play on something crazy, like +4x8, you better be badass or the synergies between minions may get rough.
I would try to use higher level characters, with a good range of their abilities.