I am going to replace my countertop and sink, but the installer wants me to buy the sink first and then he will make the appropriate cut outs for it. Then I will need a plumber to install it. 1) How do I know how to buy the right size sink so that my plumbing wont need major work? 2 ) Or, should I just buy whatever kitchen sink I like best and then have my plumber make adjustments? (If I go with #2 how much could it cost?)

Here are some pictures:
http://tinypic.com/r/u7vap/7
http://tinypic.com/r/k9gu9k/7

Thank you!