I basically agree with you, but for the fact adding elements of a more realistic madness would help me portray that odd, alien mental state in the first place. If he's a mentally healthy alien who acts like a mentally unwell human, a mentally unwell human is still a good model for the behaviour.

Which is even if he is mentally healthy, since other Time Lords think he's mad.

I don't want to start getting out diagnostic charts or anything like that, because that's not what I'm trying to achieve - I'm just trying to play with the more difficult, negative aspects of his eccentricity in addition to the quirky, cute ones, and I'd like advice for getting into that mental state.