Skip the db651s and go straight for the MM651s.
They are a night and day difference from the db/dxi series.
http://www.amazon.com/Polk-Audio-AA2...keywords=mm651
watch polk's ebay store for refurbs, I've picked up a few sets for ~$120
http://stores.ebay.com/Polk-Audio-Di...p2047675.l2563
As far as headunits, I would go for a 'nice' used one over a cheap(<$150) new one. My favorite is the pioneer 80prs.
Sound/vibraton deadining in the doors will make a larger difference than most speaker/heads unit will.