As far as I am aware, a diagnostic check is only going to tell you the condition of the engine and modules, and if fitted ABS electrics. It won't tell you the condition of the overall wiring harness and associated connectors.
They all deteriate through corrosion. Some of the wiring faults symptoms can be obscure, but usually relatively easy to track down. Most novices can usually handle simple wire repairs. Sometimes it might be as simple as cleaning the corrosion off a connectors pins and sockets.
The truth is you may not have any problems, though any make of bike can give you wiring problems at this age so I wouldn't be put off buying it unless you thought it was a basket case. I'm not sure looking under the seat will help much. Sure you can look in the fuse box, but most of the electrical connectors are under the tank at the headstock. They don't flex so any problems there is going to be corrosion.
Generally speaking if you can post the correct symptoms of a fault, there are lots of people on this site who will guide and direct you to a successful conclusion.
Ian