No bother Dan, You can get those convertors in Halfords, they are made by a company called Autoleads, but this is only needed if your keeping all the factory speakers and just adding and amp and sub.
If you do decide to upgrade to a new Headunit, then it is worth while upgrading the speakers too, since your not doing it yourself, it shouldn't be much trouble to a professional.
A lot of guys seem to like infinity speakers, i have a mix of InPhase, and Pioneer and am really happy with the result.If your not planning to enter any sound off competitions pioneers are cheap and cheerful. Amps i have a pioneer 4 channel amp running the 4 speakers and InPhase amp running the Sub, have a look in Halfords, they generally have a sound wall with various system set ups. Make sure you dont let on you dont know much or these audio installation guys will take you to the cleaners. Any questions you have ask them here first.
SE models have tweeters as standard, not sure if you can re-use these if you upgrade the fronts, im sure someone will be along soon to correct me.