Hi Georg
Any old manual focus macro lens from any of the major players will work just fine. Minolta, Contax/Yashica, Olympus, Nikon, Canon, etc. The thing about macro is that you really dont need Auto Focus at all so those old lenses work just fine. You may view my Small World macro folder on flickr here.
http://www.flickr.com/photos/8539414@N07/sets/72157626866276690/
All photos there were either with a NEX 7 or A7 using some type of old lens, some not even macro. Many times I use a favored 50mm (Yashica ML 50/1.7 or a Minolta 50mm) with either an extension tube or just a Minolta Close Up Lens No.2. Or sometimes a combination of all of them. Thats the great thing about macro, you can play around with many different techniques to see what results they yield.
You can see that I dont shoot at crazy magnifications, usually around 1:1 or a little less. All my shots are handheld with no flash and I find that anything more then 1:1 really requires a tripod and some type of lighting. For me that is way to much to lug around. I prefer just my camera and a couple of lenses so I am free to follow the bugs.
As to focus stacking I dont use it myself. But I have seen some wonderful results with it. My personal belief is to simply capture the image as you find it and do as little digital trickery as possible. Seems focus stacking is really moving away from pure photography and into the realm of digital computer imaging. But then I use color toning on my photos all the time and that is pretty much digital imaging as well huh? People in glass houses....
@HiredArm- I can definitely recommend C/Y glass. The Yashica macros are excellent lenses. They made two different 50mm ML Macros, a 2.8 and a 4. I own the 4 and it is a sweet little Tessar lens. Only downside is a 5 bladed aperture, but that is no biggie. The newer, faster f/2.8 is an amazing lens also. You cant go wrong with either one. I have no experience with the 100mm Yashica Macro, or the Zeiss Makro Planars, but they are supposedly superb lenses as well.