Have you considered slapping this guy? This isnt the only sub with a built in amp on the market. As a direct competitor, you can also opt for the JBL CS1214T. However, pre assemble subs are good for 1 thing only, space utilization. The only valid point he made was you can get another sub in a built box for a sedan's trunk.
The ready to install L ported boxes available in the market are not tuned. If you are going for a spl setup, get a sub, build the right enclosure and watch it destroy a much higher rated sub in a generic box to pieces. If you are only getting the sub, the right kind of amp would be a mono or 2 channel on bridge mode. Just watch out for the RMS values of the amp. It should match or exceed that of the sub inorder to run that sub to it true potential
And since you have a saloon, your bass will sound even better without your trunk rattling all over the place. Get 1 or 2 sound deadening sheets, cover the trunk and boot area and enjoy those highs and lows. Not the krrrrrrrr krrrrrrrrr and the clipping sounds