Good sub.
I would first determine what your goals are and how much you want to spend. Factor in labor if you don't plan on doing the work yourself.
If possible I highly suggest going all active so you have the best chance of getting the best sound out of your speakers.
For the sub, I don't know of any under seat boxes for the Lancer, but you might be able to get one custom make. Better yet I would have a custom made box corner loaded in the trunk. Save under seat for a processor (if you go with one) and amp/s. Also could put everything in the trunk.
For the amps I would go with one of the compact D type amps. JL and Alpines are good and you can get them pretty cheap of Ebay or CA forums.
Really a million different ways you could go, just need to figure out where you want to start. Any good shops in your area?