The 8400A is very old/large/heavy, but a nice meter if you like Nixies. AC and ohms are option boards, so may not be included. I'd say $50 is on the high side.
The 8842A is similar to your HP 3478A, but has a nice readable VFD in place of the 3478A's crappy LCD. $50 is a great deal if it's working.