Please, please read this and give any advice you can give!!!

[B2r3y]

As for getting a new battery and a used alt. you should be pretty well off for now but i would get a bigger alt. soemtime in the near future. The fact that its used i personally wouldnt like but i guess its yourthing hehe.
Now for your gauge dropping and moving around when your driving wiht your heat and lites and radio on, you might just have your battery 1/2 charged or not fully and your used alt. isnt fully charging it. Mine only moves like that when im running alot of power without the car on and i start it and it will usually move around for about a day or 2 while i drive untill the battery is fully charged again.
Stick a charger on your battery and let me know. Hope some of this helps.
Bryan