[Koha] Advice on MARC21 fields

Hello everybody, I write hoping that you could help us with advice about MARC21 configuration. My name is Rodica Balasa, Project Manager at Nemesis IT from Bucharest, Romania. We are helping IMAR (Romanian Institute of Mathematics) to install Koha for library management. Currently the library catalog is recorded in writing (no database), so it is important that the MARC description of most of the books to be downloaded from the z39.50 public servers. We arrived at the point of configuring the MARC21 control fields, and as I understand that the choices we make are very important, I thought of asking for advice other libraries that have more experience with Koha. Thank you very much! Rodica Balasa