I'd go along with what SOPS says. Emirates recruitment know all about the prep companies and will be able to tell if you have handed over your hard earned dollars (or pounds or whatever) to one. That won't stop you getting the job but it is not what they are looking for- they want to see the real you.
Preperation wise, they want to see some evidence that you have researched the job and the country because they come as a package. You can't separate working for Emirates from living in Dubai so make sure you know as much as possible about both the company and the city. Don't, for example, turn up for interview without knowing the seven Emirates that make up the UAE. Know a b it about Islam. The five pillars would be a good start, and know when Ramadam is and what it means. All this stuff will be useful anyway if you do end up in Dubai. It goes without saying that you need to have a good knowledge of the aircraft you are flying now and ATPL theory.
What they are looking for at interview is somebody who can not only fly an aeroplane but who will fit in at Emirates and in Dubai and won't be complaining on PPrune after 6 months that it isn't what they expected.
If you have the time it is worth coming to Dubai for a holiday before the interview. If nothing else you will know what you are getting into.