You wont buy quality pH pens at the local Wal Mart or anywhere else.

Tap water is perfectly fine. If its water, the plant uses it.

I too do not use distilled water as every single electron has been boiled off. In fact, try this...

Drink nothing but distilled water and see if you live for 2 weeks.

You wont.

You cant.

Plants are a bit different but still, tap water is fine. Let the water sit as said above, for a day to let the chlorine gas escape.

The preferred pH of plants is strain dependent. I personally use as close to 7.0 as I possibly can. Continued watering with acidic water can cause problems with soil becoming too acidic...leading to other issues.

A neutral 7.0 will not have an effect on soil...just the plant....and you wont get nute lockout as easily.