First of all what size is your stock alt? I would do the big 3 first (0ga) cause it wont hurt anything doing it, plus if you do get an alt u'll already have that done. I would do the big three and replace ur stock battery with one thats better suited for car audio like XS, Kinetik, Optima YT to name a few and then i'd add one in the back also, if you still are getting major current draw ( i.e. lights dimming, bad voltage drop) then i'd start to look at an alt, like DC or Mechman, i had a blazer with a 200a alt no big 3 (this is b4 i knew what that was) a YT up front and a Stinger Batt in back and i had no more light dimming issues what so ever, idk how much drop i had though cause i was more noobish back then ,than i am now. Even replacing ur batts and doing no alt u are most likely gonna have to check (and charge if need be) you batteries to keep them fresh.
Simply
1. Big 3, 0ga is a must do it right the first time.
2. Replace stock Batt and add another Batt in back.
3. replace alt if you contiue to have problems with ur electrical.