I flew from Sicily to Bahrain on an Aloha DC-8. But it's perhaps too similar to the 707, and there is another new DC-8 on the market. (Don't know how good it is.)

DC-9 is not Boeing, and there is a very nice new DC-9 on the market, so I don't think it would make much sense.

The classic 747 would be perfect, and would complete the seven series (almost). There's not been a really good classic 747 for FSX, and I think it would be popular.