85 amps is still a pretty small alternator to be sure, I would'nt worry about any repersussions about the battery being another load....
While tirefyr is correct about the load issue, in actuality it wont have any affect on your car stereo or electrical...
For instance, Steve meade (here is a link to his tahoe)
http://www.sounddomain.com/ride/523384 has a 10k watt daily driver... He also has 2 h/0 alternators.... Those alternators CANNOT keep up with the amount of power his amps require... so what does he do??? Well he has 6 of these batteries
http://www.batterystore.com/Odyssey/PC2150.htm
Bottom line batteries are better than caps, AND your alternator will not have any trouble keeping both front and rear battery charged. I have a 2nd battery in the rear and I bump it ALL the time, of my stock 70 amp alt. Trust me you'll be fine.
Go with a battery, you'll be glad u did