Very much this. Spend your entire component budget on some better quality fronts.
Alternately, if any of these Q form kicks will fit in your burban you could do a set in your stock front location and a set in the kicks with good results:
www.qlogicenclosures.com
Have you decided how much space you want to sacrifice for a box? Can you build one yourself? Have you budgeted for this?
I do not believe you want to go much larger than a 3000W amp if you only plan a HO alternator and even then it'll be pushing things pretty hard. Pair something like that up with some of the new Adire Brahma subs, those get loud and sound good along the way and are a great 1200W sub. Would be about my top choice if I were buying new in that budget.
I've got a very nice Focal 4x150 amp for the fronts if you're interested at 400$. Fresh refurb from Zed Audio.