Drilling Stainless requires a bit of care, but should not be a big issue.
1.) Drill a pilot hole for the step bit, to help keep it on center.
2.) Using plenty of pressure, and going slow to allow the cutting action to work, drill.
3.) After drilling, sand the hole with a bit of sandpaper. (not Steel Wool) This removes the burrs.
4.) You may consider a weldless fitting for your ball valve. I have been using one for well over two years now and I have had no issues with it.