The Doria is a piece of cake on mix. It is some of the boat operators and guys who lose their marbles for the sake of a bit of old pottery that are the problem
Read over Gary Gentile's books on the Doria, don't dive it at night and you should be fine. Include a gas analyser so you know what is in those tanks
. Gary is a real gentleman: what he says in his books on the Doria is trustworthy and gives you the score.
I disagree with Dave on this: having a reel is quite useful. It allows the chase boat, if there is one, to follow your SMB as you drift off towards Africa in the event you could not find the shot line. I do not disparage your skills: it simply does happen, no matter how perfect you are. If no chase boat, pick another boat operator.
Joe's advice is spot on, by the way. Bring a Jon line. Also Charles (Solodiver): bring enough bail out to deco on your todd.
To go inside, get formal deep wreck penetration training, and you should have done a lot of this before. That applies to any wreck, but especially the Doria because it is falling to bits: it should not be the first deep wreck in which you plan to spend 30 minutes inside.
Alex