As long as the broken glass door doesn't affect performance I wouldn't worry about it. If you decide to upgrade the deck, look for a Tandberg TCD-3014 - These are among the finest cassette decks ever made.
My 2nd choice would be a Bang & Olufsen Beocord 9000 - these are *phenomenal* cassette decks. B&O worked with Dolby Labs to produce Dolby HX-Pro which is an encode-only system that imparts incredible sound quality to cassettes. Tapes produced with this can be played back on any Dolby-B capable deck (virtually ALL cassette decks) and you will get the benefit of the HX-Pro encoding.
If you really want a top quality cassette deck, look for either the Tandberg or the B&O that I mentioned. They are head and shoulders better than anything else out there.
A good 2nd choice would be the Harman/Kardon Dolby Hx-Pro decks from the early 80s - not as good as the Tandbergs or B&Os, but damn close and a whole lot cheaper...