280s only a bolt on fit if your car is late 88 onwards with 256mm brake, before that they had 239 brakes. With the 256 the calliper carriers unbolt from the hub, 239 carriers are cast into the hub. If you do have 239 you need 256+ 4 stud hubs, Passats are a good source as are higher spec SEAT models from the 90s.
As for the 280mm carriers, MK3 GTIs are popping up in scrappies all the time now and are a good source, just make sure its a pre-96 model. Later SEAT models also got them, as did various high end VWs. Ideally take the 280mm callipers as well, you can get away with 256 callipers but you need to use part worn 16v pads and/or grind down the slider pins a bit.