[Nyxeris]

They'll plug and play, you just won't get the full benefit if you don't have an amped system as the L7 speakers are all 4ohm so take more to drive.

If you have base audio as it stands (no tweeters in the front doors), you'll need the rear parcel shelf trim too. Usually pretty cheap though, mine were about £10 a side IIRC.

If the car is local, is there any hope of getting the L7 amp and wiring too?

 

[Nyxeris]

Should be a sub under those grilles, and on both sides if your car is pre-2009. After 2009 it's a bit of a gamble – you might have one under both seats, you might just have one under the driver's sear, or you might have one under the passenger seat but not wired in.

JBL kit isn't standard/original, might be worth pulling a door card/sub grille off to see what the speakers are in there? If someone already did the wiring for you, an aftermarket setup might be much less faff!

You need the fibre optic cable from the head unit to the amp, and the power wiring for the amp at the least. Not actually certain, could be worth searching for a full retrofit thread to double check as I'm not 100% on what's necessary, looked at it a very long time ago now!