That's a relief.
Well, if you're going with one Sundown SA-15, then you'll want an amp that will put out 600-1000+(I'm not completely familiar with the SA subwoofers, but I know people run them at double their RMS power rating, which is 600W). Your next decision will be at what impedance do you need that power? Because the Sundown SA-15 is a dual 2 Ohm driver, you'll have to wire it at 4 or 1 Ohms, so look for an amplifier that puts out 600-1200W RMS at 1 Ohm(this depends on a few things; your box being one of them).
If you end up buying a Sundown SA-15, that's $235 at the SSA store with free shipping(links will be at the bottom of the post). If your budget is $1,000, then that leaves you over $760 for an amplifier, box, and wiring. As for the box, you'll want to get a design from somebody, then either build the box yourself, or hire someone to do it for you. Do NOT buy a box off of
eBay or the like. Box designs are highly dependent on the driver you're using, so if you want the best performance, go custom made. Wiring will be $60+, and possibly up to $100, depending on the brand, and how high-end you want to go for RCA cables and speaker wire.
As for speakers, the ones in your kit are probably sensitive enough to stay fairly loud with a single SA-15 on 600-800W RMS. If you end up wanting to upgrade, make sure the speakers you purchase are fairly sensitive(the rating you will most likely see will be marked as "SPL"). However, there are different methods of measuring the SPL rating, so you might want to search for the differences. The other important factor about the speakers you get will be the RMS rating. Your head-unit is capable of only pushing a few watts per channel to your speakers. If you buy speakers with a 150W RMS rating per speaker, you might not(probably will not) get the volume you're looking for.
Because of this, if you want to upgrade your speakers, you'll likely need a second amplifier. Based on my Google search of the head unit you have, it only has one pre-amp out(RCA out, on the back of the device). If this is true, your subwoofer amplifier will take up the only set of pre-amp outs you have, which means you'll have to purchase a subwoofer amplifier that has another RCA out(RCAs are the cables that carry your music to the amplifier). Consequently, you will no longer be able to fade your sound to the front or back with the head-unit. To overcome this, you can buy a different head unit with more pre-amp outs. There are other features you're looking for in a head unit, but if you don't care about fading, then don't worry about any of this.
Now, for actual recommendations... I'd say that the Sundown SA-15 is a fine woofer, and will be much better than a Kicker L7. I think it would be a great starter subwoofer, and if you end up getting bored, you can pick up another for fairly cheap(or, you can upgrade to a single woofer with a higher power rating). If this subwoofer seems like the right choice to you, because after all, it's your choice, then I suggest looking at the vendor section of the forum for a design.
I know Buck can give you good, to the point, cheap designs. The other designers might be able to give you a more comprehensive rundown of what's going on with their design(only because they might know more about certain things from an engineering standpoint). However, that's not to say that Buck won't give you a working design. As for the box itself, that's entirely up to you. There are box builders and shippers on this site, or other sites, and I'm sure you would be able to find someone local. If you can build the box yourself, that's obviously the cheapest route.
And as for the speakers, I suggest getting the Sundown, choosing an amplifier(remember, you'll want to decide on what RMS wattage you want to run), getting a design/box, and installing all of it, before worrying about your speakers. At this time, you'll also be able to detect rattles that need to be stopped, so you'll be able to decide if and how much sound deadener you want.
Links:
Sundown Audio SA-12 600 watt sub woofer
SSA Store - SKAR Audio VVX Sub woofer