It's a bit of a crap shoot. If the guy has service history and the bike looks taken care of it might be a solid runner for a very long time.
However, I've owned three Ducatis. The first a used one. I personally will never buy a used Ducati ever again.
As indicated, if maintained the aircooled 900 lump is pretty bullet proof.
Take some deatiled phots and post them up. I'm sure someone will pipe in with info.
Good luck.