My preference would be for the earlier 2 channel models over the later single channel ones - They are just easier to play and control with an A-B switch in front, volume wise, over switching the boost and distort in and out.
$300? Hmm... it would have to be in excellent cosmetic condition, with all original knobs and no scrapes on the control panel and no rips in the tolex. Rust on the corner pieces usually can't be avoided.
Normally, although not always, a good cosmetic appearance means the amp has been well taken care of. Then again, I have seen beat up Concerts that have sounded superb.
But for $300, it had better look as good as it sounds.