Go with a 4 channel amp like the Kenwood XR-4S which you can buy for under $200 refurbished. 2 channels for the stock underseats and the other 2 bridged to the sub. Go with the corner enclosure and put in the best sub you can afford. Alpine Type R would be a very good choice if the enclosure is at least 0.75 ft2. Type S may also work, but the Type R has a much better reputation for SQ. If later on you feel you need more power, the 4-channel amp can be used to drive the front speakers, and you can get a mono amp. You're definitely not going to upgrade yourself into a corner this way!
Read up on the MS-8, it works wonders with the L7 speaker setup!
