Right,
Firstly the SE and the Sport have the same stereo. The S is the one without the amp.
If you use the guide i linked to that adds an RCA output on your standard Amp (hidden behind the glove box to the left) so then you can just run RCA cables from the standard amp to the amp for your sub. So from what your saying thats the easiest option for you, and keeps your stock headunit and speakers, but gives you the abilty to add an Amp and Sub.
If you look through the Audio section it has been explained Many many times how to change the stereo and how much work it is.
Thirdly the link IRLRS200 put you too is a new thing that will let you change the headunit for an aftermarket one with out changing all the speakers and that bypasses the stock amp. You could then use the RCA outputs of the aftermarket stereo to power your amp and sub. So you could do that but you'd have to buy the Leads, and an aftermarket headunit as well as your Sub and Amp.
Again Hope this helps.