I just ordered directly from Kerscher the 17mm balljoint - Euro39.66 per piece (the price shown in the webpage is not update). Just send them an email
Tuning@Kerscher.de
The other part for the front you may consider is the anti pump steering kit because the steering arms of the 944 units are a little bit higher than those of the 1302/03. So you need to modify the 944 steering arms so that the tie rod balljoints go in from the bottom rather than from the top.
For the McPharson struts, you need to get the late 1303 units or buy some aftermarket items such as coilovers from Kerscher. Besides the 944 parts, I think these are the most expensive parts for the conversion. Make sure that the 944 Turbo spindles will fit before you do this (you may need to modify the 944T spindles or the strut mounts).
You need to buy some handbrake adaptors for the 944 brakes from Kerscher as well, or you may fabricate some yourself. I am not sure about the T1 transaxle output flanges for the 944 CVs. You may need to buy some aftermarket T1 output flanges with T2 bolt pattern. I haven't pay too much attention on the handbrake and output flanges because in my case I will not use the handbrake. For the output flanges, I plan to use Quaife LSD with Type 2 output shafts which will readily fit Type 2 flanges and 944 CVs.
You may also consider to change all the bushings to urethane items but these won't cost you much.
Don't forget the late 1303 front control arms and the front swaybar (a stiffer 7/8" swaybar will be good).
Tim