While you have it all apart I wouldnt replace just a couple id do them all as the other ones will break once you put it back together most likely anyway,to buy all the studs and nuts gasket etc etc cost me around $220 from Nissan from memory, most workshops I went to wanted anywhere between $400 - $800 to remove the broken studs and install all the new ones, I ended up paying a mate $100 to do it for me lol.