My 2003 1.4tdci Zetec MK6 fiesta has Central locking but not remote central locking.

After browsing the web (including this post) it seems I can add a GEM which has remote central locking and program a key from Ebay and bobs your uncle.

Has anybody done this and is it as easy as it sounds?

I've also only got 1 key for my Fiesta and would obviously need to program the new remote key fob to work the imobiliser.

Having a look here it says it can be done without using the dealer but I have to use both keys of which I only have one.

Could I program a new key using the method above with only 1 key?

Thanks for any help in advance.