Using all chain may not be preferable for your size boat. The chain will be more expensive, a lot heavier and will take up more locker space thus reducing the max length of anchor 'line' you can handle.
The recommended length of chain varies by application needs, type of anchor and hull size.. Some folks state that your chain should be at least at long as your hull. I think it is a bit more complicated so do some research. Pick your anchor first and look at the anchor manufacturer's website for guidance. Probably you will be reusing an existing anchor. In that case look for similar new ones of the same type for guidance
Regarding the electric cable you found; If your hull is older, it may have originally had an optional windlass that had lower current draw requirements than newer models. For example, a new windlass may require 60 or 70 amp service and breaker/cable wiring but the existing circuit is a 40 amp design. In that case, you will need new cabling, both 12 volt feed AND ground wire back to a new breaker at the battery.
If you existing windlass wire gets power from your fuse block, remember that you can not draw more amps than your accessory breaker is sized for. In addition, that breaker has to handle everything else that gets power from it.
Knowing the wire gauge of the feed you found can help determine it's max amp capacity. Use the 10% drop table but note that the cable may be rated for higher current than it can handle depending on the ground size, breaker or fuse sizes and other shared loads.
