speakers > AMP > Head Unit
In that order.. of importance.. Speakers or investing in good set up speakers will make the biggest difference.
IF me I be looking force this in place.. http://www.crutchfield.com/p_091165V...30.html?tp=106
but getting on ebay for cheaper.. http://www.ebay.com/itm/New-Focal-Po...ht_5122wt_1144
One these two Cadence Amps 5 channel power all front/rear and Sub http://www.woofersetc.com/c-29-multi...facturerid=121
(one build quality better then other.. I went with cheaper)
Or get MB Quart and just power front and bridge rear for subwoofer MB Quart ONX4.125 - 1000W RMS ONYX 4-Channel Car Amplifier/Amp
(i put this one in my brother GF car, works very well, 2 channel front and bridged rear)
Had both amps.. both pretty good unless you want to wake neighbors then by all means get a Mono block and small 2 or 4 channel from Amp for front.
MS-8 is best but they are expense. Otherwise just line converters. MS-8 resolves the bass getting cut.
Again eBay direct from JBL w.warranty http://www.ebay.com/itm/JBL-MS-8-Mob...#ht_3509wt_910
$270 Focal Front speakers
$200 for AMP (need wiring kit)
+ subwoofer you be on your own
1k+ for a pretty decent system
Removing the stock deck seems be a pain and OEM deck isn't worst deck.. and cost of a good deck is expense.. maybe have no sync then worth it