alternator upgrade and gauge help

You should probably fix the low voltage conditions you have now. Just throwing more current through those weak connections will cause big problems, maybe even fire.
Now about what you should have... The system was sufficient for standard equipment when new. In other words, there was enough current entering the cabin and passing through your amp gauge to operate stock equipment. Lets get it back to that condition.
If you upgrade to headlights that draw more current, add relays to supply that current and pass only the small amount of current to energize the ralys through the cabin, amp gauge, headlight switch, and original wiring. Same applies to electric fans and other high draw equipment. Upgrading the stereo head unit isn't an issue. Adding a amplifier requires a relay, alternate power source, larger wire.
Keep the large gauge wires and high current out of the cabin.
This is how newer vehicles are wired.
If you want to learn, I suggest you start by repairing what you have. Then do a headlight upgrade. Plenty of info online to help with that.
Afterwards you should have a good understanding of everything in this post and how to continue with other upgrades.
As for the alternater to supply the additional power for your upgrades.. I cant help you with that. Others here can though.
Good luck