well, if the sub is 800 rms, just dont get the cap, but it sounds like wit the 2 15's ur gonna need a HO alt later on for those. for your one sub now, depending on the VC's, just get a a bahn a8000v or a8002v
Yes John Jimmy, you need an inverter to convert your rear speakers signal/wires into an RCA output. Then need to run REM(remote turn on wire) to acc spot in your vehicles fuse box so every time you start the car the amp will turn on. I would suggest just to get an aftermarket HU because of Sub outputs and other features that are way better than stock radio. But yes it is possible.
I'd guess that sub is probably 350-400W RMS from what I could find on the web. Like everyone else said, you don't really need the cap. You will however, obviously need a box unless it's a pre-loaded sub/box package.
stock electrical will handle that sub but get a good amp thats powerful enough to push around 400rms in that sub at what ever ohms that sub is but make sure its a amp that does more power at what ever ohms your other subs are going to be configured for just so you dont have to buy another amp later when you get the 15's you plan to buy ( if this is possible for you, just because it would be the smarter thing to do IMO )
for example:: if the sub you have now is 4 ohms SVC get a amp that does around 400rms at 4 ohms but also does around 1000rms at 1 ohm just so in the future you can get two 15's that are around 500rms and 4 ohms DVC then wire those subs together in Parallel/parallel to present the amp with a 1 ohm load thus getting the full power out of the amp without haven to buy another amp when you get the new subs