Hi TaeJung:
Nagra T Audio can handle up to four different speeds, separate EQ configurations, and record tape bias'. You can see how many adapter cards it has once you open the card cage door and look at the both Audio Cards for CH1 and CH2.
I'll have to verify the source of the Error 7. However, if it is indeed the memory back up battery, all the fine adjustments and settings are lost unfortunately. The battery can be changed but you would need to set up the machine again.
Generally speaking, out of all the master recorders I have gathered so far, the Nagra T Audio is one of the most beautiful and nicest operating recorder without considering its factory sonics. On the other hand, if it develops any issues, it is very difficult to repair them. I know of only one person in US who is equipped and able to fix the Nagra T but he is not cheap to say the least.
So in summary, I would not recommend you buying the machine unless the seller can fix it first to display no errors and have enough adapters already installed that matches your tapes you plan to record.
Ki