Markus, go for it, it's a lot of fun!
The Sigma Art lenses are crackers. I have the 35mm and there is a lot of vignetting wide open, but it collects light insanely quickly
Take lots and lots of subs and you should be able to get something nice with stacking. The non-tracking tripod will do natural "dithering" and help with the noise, but I'd recommend taking some flats afterwards to help out with the vignetting.
I'm a bit of a pixel peeper, and up around the celestial equator wreaks all kind of havoc with star trails...with my 35mm, I found about 13 seconds was the max I was happy with. I'd expect to see star trails in shorter exposures with the 50mm. But, you might be surprised what you can get from ISO1600 or 3200 in 8-10 seconds
