I'm totally new to this (both the bulletin board and plumbing) but here I go....

I am replacing my kitchen sink and faucet (as well as cabinets). My problem is that I believe that the sink trap and pipes may be lead all the way to the stack pipe (vertical pipe). What's also interesting is that the stack pipe below the kitchen sink is copper pipe but above the kitchen sink, it seems to be lead. Some plumbers that I talked to over the phone said that if the sink pipe is lead, to replace the sink properly, I would have to replace a lot of the pipes back to the central drainage area (and dig up my walls). If the stack pipe near the sink (and below the sink) is copper, my feeling is that this would be unnecessary - so my questions are:
1) how can I tell if my sink trap pipe and stack pipe above the sink is lead or cast iron (it's definitely not sounding like copper when I knock it. When I scrape it, it's a bright orange-ish colour)?
2) If it is lead, I will need to replace it (none of the plumbers are willing to work with the lead pipe - they will only replace it), so:
A) Would I just need to replace the pipe to the copper portion (which starts on the stack pipe below the sink)? and
B) Would I also need to replace the potentially lead stack pipe above the sink?

Any advice would be appreciated. Thanks and Happy New Year!