hehe, i cant just tell what makes a good system, but i can tell u what to expect to run a good system.
You will need sound deadening to keep your sound in the vehicle as much as possible. You will also need something such as Rattle Pad put in behind all interior panels to prevent rattles.
You may need spectrum sludge on the outside of your ride to prevent things from "shimmering" that is causing a nuisance.
You will need, on average, enough batteries aH rating to meet half of your amps rated pull rating.
So, for example, if you wanted to run 2 5,000w amps, that's typically capable of pulling 1,000A, you want something around 500aH worth of batteries for daily. That would be 5 D3100s = 585aH
You will also need a 250-400A alternator or dual 200-250A alts if that's possible for your ride.
You will need plenty of 1/0 or larger runs to suffice.
The cost to build your box is most expensive when doing bracing and\or multi layered sides.
Also, if u do put all this in your ride, you will also have to invest in a NEW suspension system. Most vehicles weight capacity is only 800-1450lbs.
5 D3100s = 390lbs
2 high end subs - 100-140lbs
2 amps ~ 60lbs (Atomic 5k's are 29 apiece)
box ~ 100-250lbs
sound deadening - 100 - 160lbs
weight - 730-980lbs.
then you gotta add the weight of how much YOU weigh, how much gas is in the vehicle anything else you are hauling around and any passengers.
To fix this, you need coil overs for the rear if ur ride supports them OR air ride suspension. Coil overs are cheaper but I chose air bags
Either way you go, you are lookin at $1,000 - $4,000 for just the bare minimum parts minus labor.