Ok I kinda hate to chime in here but HU is the most important part of a systemLook for these specs
1. 4 volt pre out
2. Three sets of RCA jacks this should have a sub output
3. Try to find a HU that has as close to 100db signal to noise ratio as possible over 100 db is better
4. Now a days most most people listen to music with a MP3 player or a phone. So look for USB or 3.5m jack input
Even if it cost a lil bit more money you can't go wrong in getting a good head unit, a bad HU a bad sound do not by any off brands
Stick with pioneer,kenwood,aphine,jvc and so on. Do not think about dual, boss, or any brand that used to be good back in the day that is now made in china and now makes head units that all look the same.