Lemon juice works because it has citric acid in it, but not nearly as much as using straight citric acid. If lemonaide was the best thing to use then Mercedes would have said so.
Lemon juice is only about 5% citric acid and the best concentration for cleaning is 10%. Even if you completely fill the radiator with lemon juice it will only be half strength of what you REALLY want. If its not very dirty then Real Lemon would be OK probably even better than most of the stuff sold in the auto stores. But if you've got some known rust and/or scale to get rid of you need bigger guns.
Use the info in the thread links I gave above to locate some REAL citric acid and it will work a lot better and cheaper. I think 2 gallons of lemon juice costs more than a pound of citric acid. Ascorbic acid isn't nearly as effective and I don't know where to buy it by the pound even if it were.
I use this place because I can drive down and pick up a 5 pound bucket of the stuff for $18. That will do 2 1/2 engines worth of cleaning. Its also good for nuts, bolts or anything else that gets rusty. I think AHL will ship if you can't find it anywhere else. You can get it at the local Mercedes dealer but its about $30 for enough to flush ONE radiator. AHL - Products - pH Testers and Supplies
On that page they describe it as a gardening supply: Citric Acid - Powder used to lower pH of water or nutrient solution.
I maybe wouldn't ask them what their customers grow with this stuff.