I've repaired a bunch of the meters. The buttons can't really be fixed unless there is soda or something in them, which is unlikely. Many 3455a meters have sticky buttons but still function correctly. If it powers on without a self test code, then most likely the meter is functional. Many of the units I've repaired hang at power on with just a few buttons lit up or all ranges display as OL. Having one that at least tries to display a number is a great start. I've contributed a number of repairs to a thread a while back. Search on 3455a in the repair forum.
I have 4 or 5 functional 3455a meters. I have a black face one I use often. They are great old meters and I find them easy to repair if you have more than one as you can split them internally. Even a calibrated 3455a is only worth about $175, max, in good shape, because the odds of it holding cal is low as these meters are getting old and prone to failure.
I buy broken ones for around $40 plus shipping, usually like $20, and treat them as a puzzle to pass the time. The 3456a is a great, stable meter for its age. I love the large, bright, red displays on them.
Post a picture of your meter operating.