My gut feeling here is that, in the absence of any opportunity for a trial recording, you would be better going for small diaphragm mics rather than large (or medium) diaphragm types. The SDCs will tame unexpected acoustic reflections better than the larger types. In reality, it is a choice of the KM183s or the NT5s, and here I would go with price and plump for the Neumanns. I don't think mic self-noise will be a worry.

The big problem with using just a single pair of mics is going to be acoustic balance. How you should get both the lute and the harpsichord to balance the strings would be tricky even with sufficient rehearsal time to go through various mic placements, but with no rehearsal time, it's a place-and-hope exercise.

The other thing to hope is that you can achieve a reasonable vertical position for the mics without the musicians getting upset. I remember I had a similar situation once where an instrumental group was adamant they did not want any visual obstructions, and, after negotiations, gave me the generous offer of allowing me to put the microphones at floor level on the apron of the (low) stage. As I did not have any boundary mics with me, I could not get away with the floor position, but instead managed to fly a pair overhead in front of the group, and got a reasonable recording.

Good luck!