what do i need to upgrade?

hi i just had my system installed spent alot of money on it . and now i have issues with voltage . ive been talking to alot of shops on what to do about it and they all want me to spend $1200-2000.  some say alternator & battery, and some say  isolator & battery . i know what im doing when it comes to car's in terms of repair and mods . im just not to good wit audio. i want to do a 2nd alt. or just a higher amp alt. if thats enough and yellow top optima but they all said the yellow top will do no good . i was hopeing to get some other opinons because the shops just want to make money . the big 3 are already on my to-do list.

car: 1988 5.0 gt hatchback mustang

amp: Hifonics BXi 2006D

sub: mtx thunder 9500

wiring :  1/0 gauge all the way back,

battery: optima red top running the car and the system for now

box: ported box built to specs by the shop facing the bottom of the car to reduce rattle

*factory 75amp alt. -Just throwing it in there.

As long as the 200amper will power your audio system (all fuse sizes added together) plus having about 25+ amps for your vehicles' electricals, you'll be good.

Do your Big3 first. You may not need a HO alt if you don't use a lot of accessories at once, plus your audio system.

With a 200a alt, 2g wiring is all you'll really need for the Big3. But hey, you might as well use the 2/0g wiring as well.