Cabinet Condition

You want an amp that has not sat in 2 inches of water for very long. You want an amp that won't fall apart when it bounces around in the back of the pickup truck. Other than that, cabinet condition has little effect on sound.
Some people want a nice clean amp in perfect condition. A good amp will have been played through and carried around and will have collected beer rings and cigarette burns. A bad amp would have been left at home. An amp that is well used will sound better than one that sat in the attic for 40 years. The capacitors need to be charged up regularly or they leak and fail.

You can perk up old amps by giving them a bath: use a gentle cleaner and get rid of the accumulated grime and cigarette tar. Use coffee to get rid of the mildew smell: sprinkle unused coffee grounds around in the amp and leave them there for a few days. Glue the tolex back where it has pealed off. You can re-tweed or reupholster old amps.

Old amps need new leather handles. If the handle is in bad shape you can get a new one, they still make them.

You can get grill cloth and new chicken head knobs.

You want an amp that plays well. You are judged much more by the way you play than by the way you amp looks.

In any case an old beat up amp makes you look like you're an old pro. When someone walks in with a Blues Jr., the veterans give themselves knowing nods - "Here comes a newbie."