I've done the kolsch kit several times and it is good. It's really a faux kolsch because it uses(at least the ones that I got) us-05 and not a kolsch yeast. But it makes a good, easy drinking beer. If you like stone fruit esters, then ferment in the high 50's to low 60's. If you want it clean, try to keep it in the mid-60's. I've done this kit both extract and all grain. If I were doing the extract, I would probably add about a cup of sugar to the boil just to help dry it out a bit. Also, I wouldn't secondary this beer, I'd just leave it in the fermentor at least three weeks then bottle. Oxidation can really effect this beer since it is pretty clean, and less transfer usually means less oxidation.
As far as carbonation, I'd use NB's calculator and weigh your priming sugar, don't just use the amount they send--it'll almost always be too much. Maybe even err on the low side of the calculator.