Stevia and Sparkling water should be fine... you are still getting the full "goodness."
By the way, GT's in not pasteurized, and I have heard that some people have used a bottle of "plain" GT's as a starter.
My total cost for starting Kombucha was $10 for the capper and $2 for the caps at a brewing supply store, and whatever I paid for the tea bags. We got the "mother" for free. I got beer bottles from friends... bottles from the New Belgium Brewing company are higher quality than others, and you have to use "non twist-tops." Twist-tops will not work...
The "mother" we got was only about 3" across and pretty weak, but by the second batch, we got a beautiful, thick mother... We wait about 10 days, and the Kombucha is still a little sweet, then we bottle it and leave the bottles in the fridge. They slowly continue to ferment until the oxygen in the bottle is used up... they get really good after a few days... if we leave them too long they get a little to vinegary... but that is actually how GT's tastes... I like mine a bit sweeter than GT's.
I have not perfected how full to fill the bottles... the first batch I filled to within 3/4" of the cap... and left them on the shelf. In three or four days I popped one open and blew kombucha all over the place... lol. They were WAY too effervescent... so the next batch I made I left about 1 1/2" of air at top and refrigerated them immediately to slow the ferment once in the bottles... these were perfect, except that they were not very effervescent at all...
So. That's what I know today.
--gabe