I am looking to purchase a remote keyless ignition as a Christmas present for my wife and I am having problems finding which one, if any will be compatible with her car. We have a 2004 ML320. Does anyone know which brand might work? Thank you

Don't do it.
Too much wiring to tap into.
You will have a hard time finding someone who wont hack up your vehicle wiring.
The Mercedes are not good candidates for this procedure due to complexity of the key and ignition wiring and components.

I have seen far too many nightmares associated with theese systems on Mercedes.
I've seen shop attempt the job,and back out half way through due to the key issues, and not being able to make it work.

Say you do find a good whizz shop who is familiar with Mercedes systems and they install it, then you always run the risk of not being able to start your car if the remote ignition control unit goes down, and they do go dow from time to time.
I do know of a shop in Atlanta that can do it, but it does not say where you are from.
Steer clear.
That's one gadget I would not use on a Mercedes.
Now I could see it if you lived somewhere very cold and it may be a practical feature, but I'm warning you, you may seriously regret having someone tap into your vehicles complex ignition system.
If you had a honda or something, I would say go for it.