Karbuf,
The Track / Performance ball joint kit from Rennbay is excellent and they are a top quality supplier. The owner is a long time 944 guy. Both my son's 88 944 and my 86 944 Spec race car have used this specific kit for years with no problems. I plan on adding it to my 88 Turbo S as soon as I get a chance.
Beware that there is another kit available on Ebay with unknown origins and unknown quality. The seller ID that you referenced is Rennbay, so it is OK. You could also Google them and find their web site for details.

Best of luck with your car!