If you meant a pH level of 6 (you said pph) then a pH of 6 is hardly acid at all. I'll base my comments on the assumption that you meant pH.
Chemically pure water has a pH level of 7. Lower (down to 0) is more acid, higher (up to 14) is more alkaline. pH is a logarithmic scale - that is, each change in number is actually ten times different. 7 to 6 is ten times, 7 to 5 is 100 times, 7 to 4 is 1000 times. That means at small differences from neutral (say between 7 and 6) you are talking ten times almost nothing, whereas if you were a long way from neutral (say between 2 and 1) it would be ten times a lot of acid.
Pure rainwater can get to pH6 or even lower just because of the carbon dioxide it absorbs as it falls.You wouldn't have to worry about copper pipes with a pH of 6.
Filters aren't going to do a darned thing to affect pH. I think the man was attempting to take advantage of you to the tune of $700 worth of expense you don't need. I won't say a filter isn't desirable, but it woudn't help the pH, and the pH doesn't need helping anyway. If you did want to modify the pH a little, throw a lump of limestone in the water, or run it through a bit of concrete.