First of all, don't be intimidated by people who don't understand fasting. You might be instructing them in a few weeks because of the good results you will get (as long as you follow the MC).
Any bottled juice on a shelf (not refrigerated) is dead (it is basically just a flavoring). It has been pasturized (heated), so anything beneficial for the Master-Cleanse has been killed.
The reason for juicing fresh lemons is the live enzymes...that is what will clean your cells.
If you find fresh-squeezed, refrigerated organic juice, make sure it was squeezed that day and use it up within a day.
If you don't stick to the basic recipe for the MC, you won't get good results...so why bother.
Your body deserves the best, so don't take shortcuts.
There's no way to tell how your body will react in the beginning. That's why it is good to start the Master-Cleanse on your day off, so you will learn how quickly the tea and salt flush will work, and any aches and pains that might flairup.