You will need a DSP. You can achieve amazing sound quality from a stock deck with a good DSP. Thing is to really take advantage of it you should run active (one amplifier channel per speaker) so you can make full use of its time alignment and EQ plus crossover functions. Budget dictates all but I like the Mosconi DSPs best. Alpine also makes a killer one but it has a DIN size face you have to hide somewhere to control it where as the Mosconi has a small control center thats easy to hide. You can also look at the JBL MS8 as its a powerful unit and you set it with a laptop then leave it alone.
You can find any of those units for under $500 (and sometimes under $200) used on DIYMA.
You can find any of those units for under $500 (and sometimes under $200) used on DIYMA.