Whatever you buy make sure you buy a Vibe Fastplug Quick Release Amplifier & Sub Box Connector, absolute dogs dangly bits when it comes to sub wiring! lets you just unplug the sub safely without having to undo all the connections if you ever need to take it out cause you need the boot space for something!
and fli or vibe subs are not bad for the money, not the best but they do the job. Make sure you get an active sub tho, much easier having a sub with the amp built in as opposed to a separate sub and amp.