I do a full boil + late addition and usually use about 2 lbs. of either DME or LME while bringing the water to a boil. Some people say that a late addition isn't necessary with a full boil... they're wrong. You will get caramelization and "twang" if you boil all of your LME for 60 minutes. Hell even 15 minutes has produced a some twang in my brews. I would add the LME at flameout, then cover and steep for about 15-20 minutes before chilling. Also throw in some late addition hops and steep them with the extract, and you'll get wonderful flavor and aroma from them.
Yes, buy more DME for the starter. You can get away with pitching the starter wort into the beer, but some people try to avoid that, as the starter wort can be oxygenated. So it might be safer to buy separate DME, make a starter, decant off the wort from the starter, and then pitch mostly yeast.
It would probably be a good idea, but any light DME will do the trick. Some of the starter wort is bound to get into your beer, so you wouldn't want to use dark or amber DME if the kit came with light extract. Just try to find some light DME and you should be fine.