The 3478A is 1988 according to the manual, and they sold it into the 1990's, so good chance you'll get a 1990's vintage unit.
The 8520a is marked 1979/1981 in the manual, so a good decade earlier.
It's very rare you'd need all that automated measurement stuff.
The 3478A is probably the best value used bench meter on ebay. There are a ton of them, the service manual is easy to get, and it's a very reliable unit.
Dave.