Melee,overrun,fix.

[Imp]

There is a game guide shiped with the game or you can download a PDF one on this site. Some stuff you could just try like tank overruns.

In answer to your questions
Tank overrun - tank gets no extra bonus it is was open to being assulted by infantry once adjacent. Not generally a good idea someone could get lucky even if only have a grenade.
Fixing equipment - No repair once its broke its broke, turn time is 3 minutes approx how long would it take you to say change a tire with bullets flying around let alone fix a track.
Equipment breaking - no not really just mobility weapons crew destroyed seperatly.
Still at least they have realistic speeds to start with.

[AMX]

And the one Imp didn't already answer:

Melee - Infantry can "Cause casualties in melee" (as the in-game message reads) when attacking infantry in the same hex.
It's not a separate option, just something that can occur when attacking a unit in the same hex.