I don't know that there's any such thing as a single, unified libertarian perspective on this. There is likely a Libertarian Party position on it, but as I'm not a LP member I haven't checked.
It seems that if the state is authorized to level charges and to try the accused, they must also provide for the defense of the accused since not doing so would amount to an uncontested penalty against one who has not yet been found guilty of anything, this in the form of requiring the defendant to lay out money for his own defense. Those who could afford counsel would be "fined" summarily and those who could not would be easily imprisoned without access to suitable defense.
Of course, to allow an instrument of the same body that makes the accusation in the first place to be responsible for the defense of the accused has its problems.
As for checks upon judges' power, I agree with a previous poster that it is unchecked in many instances. But I would add that a properly understood and working jury system is a check upon judicial abuse.