The original ML deck (even the basic stereo) is already a decent deck. Afterall, it's not a Hyundai or car rental Ford.
As the car comes, it's not the weakest link in the system, the speakers are. Do the search, pretty much the overwhelming opinion as far as sound goes. Of course you can't see the other stuff just the deck so maybe sound isn't his top priority, then maybe a deck is what works for him.
Anyway, changing only the deck presents some problems for him as very few decks can drive the 2 ohm load required by the original speakers. furthermore, the drivers, cone (pretty much every part) of the OEM speakers is about as cheap as it gets. In reality, the front drivers are 4 inch speakers mounted to fit a 6 inch hole. Just by switching the front door speakers will gain in loudness, Mid-Bass and clarity while the best deck in the world will sound exactly the same using the original speakers. In fact others have tried and found exactly this result. (use the search). People who complained about exactly this heard no difference till they swapped out the speakers. Logically if your speakers cannot produce high quality accurate sound then what does it matter what's hooked up to it, you'll still get the shitty sound because that's all it can do. Ah but I digress...
But hey, it's his system, his budget and it's up to his priorities.