1) How old is the kit? Old extract can throw unwanted flavors. Old hopped beer kits might have lower levels of hop bitterness. How fresh is the yeast packet than came with the kit? If it's more than a few months old, you'll need to make a starter from it or get more/fresher yeast.
2) There's nothing inherently evil about white sugar; it's the percentage of adjuncts you need to worry about. If the LME in the kit has any adjunct sugars in it, adding an additional kilo of sugar might give you yeast nutrition problems, since it would take it well above the 10-20% sugar adjunct level at which problems like thin body, cidery notes, etc. can start to show up. Add yeast nutrient and pitch more starter than usual. If you're twitchy about the high adjunct sugar level, substitute in some light DME.
3) Your plan to add French-press coffee to secondary sounds good. You don't want to put actual grounds/beans into the beer, though, since the alcohol might extract tannins from the beans if you leave them in there for long periods of time. If you do add grounds/beans, put them in a bag so you can get them out. Another possible trick is to extract the goodies from your coffee grounds/beans by soaking them in top-end vodka and adding that to the secondary. Randy Mosher's book, "Radical Brewing" has lots of ideas for working with coffee.
4) Extract kits don't need minerals. Depending on your local water, you might get better results using RO/distilled water than typical dechlorinated tap water.
5) If you're not hopping, then you only need to boil the wort to sanitize it. If you are hopping, do a normal "wort boil" with the added sugar/DME and water you're going to use to dilute the kit, but only add the kit about 30 minutes before knockout. If the kit is old, you might need to add some bittering hops get the IBU you need. Of course, tropical stout, which is what you're trying to make, isn't really hoppy for a stout.