At some point in advance of the aircraft traveling to pre purchase there should be a complete and thorough cockpit sweep and avionics functional check (it is a good idea to have a pilot and maintenance person work together here). Preferably this should be done far enough in advance to address any major issues that are found by exchanging or troubleshooting expensive avionics boxes or equipment. I see many discrepancies written by facilities during pre purchases for minimally functioning systems that in a day to day operation would be allowed by either MEL or the understanding that it is an intermittent problem and not yet identified. These can be the most time consuming and expensive issues in a pre purchase to repair.
Any item not used on a daily basis such as HF radios, phone systems and heads up displays are prone to small issues that can become a concern by someone not familiar with the aircraft and must work correctly at the time of the sale. It is important that each item or installed system should have available a dedicated manufacturer’s manual and most likely a current flight manual supplement. From that, find the documented self test or operational test for that equipment and complete it step by step as that is what will happen at the facility. These tests are seldom done nor are they truly required on a daily basis and can uncover problems that go unnoticed in normal operations. Also check the cockpit for burned out light bulbs, loose switches or knobs, crew seat operation and all required emergency equipment. Here again, many of these items can be fixed simply in house but would require additional labor and undue exposure at the pre purchase.