Do not!!! I repeat: Do not!!!! take in someone from an advert!!! An agency is your best bet. You have a trial period where you can return/replace. You will also have to put down a deposit (6,000 AED?) with the government and process/pay all sponsorship costs.
If I were to have a housemaid my preference would be Indonesian. Problem for you would be that they are Muslim and pork is a no-no - can't handle, can't cook it and they generally work for Muslim families. Work with benwj's criteria, except I would go with fugly, rather than just plain ugly.
(eliminates any temptation
You are responsible for them, even on their days off and they are out of the home. You will also be wise to hold onto her passport. Terrible I know, but necessary. You may want to purchase a baby cam from the UK so that you can see what is going on when your child is in the care of the person you bring into your home. If something pops up, you can't use it in a case against the housemaid, you can only threaten her with it and get rid of her.
Keep in mind that these "nannies" are not nannies, but housemaids and have no childcare training/experience other than what they received from previous employers, or if they are brought in and you are their first placement, they have no childcare experience. They don't come close to a proper Nanny/Au Pair.
As Chocs said, check expatwoman.com. They have an entire section devoted to maids. Alot of info about maids and you get to see the good, the bad, the ugly, as well as expats who are looking to rehome their maid - which is probably the best source.http://www.expatwoman.com/forum/topics.aspx?ForumID=42