You should get a general idea of what you want by answering some questions about your project. How much money will you want to spend, what are you trying to achieve (more counter space, more cooking space,etc.) Then talk to architect. They will know what the plumbing and electrical requirements...
If the guy is waiting on more money, he should have charged you more. Maybe if he did grfeat work, he would like to get a written reference from you. If he is just getting started, this would be worth far more then money.
Assuming that you have power to the fixture and you cover the sensor and still no light, check to see if you have corrosion in the fixture. If the screws are rusty and everything looks corroded, it is probably cheaper to replace then repair - also much simpler.
Like warden said, once you have located the position of the opening, drill holes in the corners. You can use regular masking to cut down the chatter of the saw blade. Also use a fine tooth blade in your jig saw and take your time with the cuts. Slower is better.