Ahh, a virgin. Well, the first time you listen to a movie that really works the low end, it'll be life-changing. Well, OK, maybe not life-changing, but it's quite an experience. Actually going from nothing to two F18s really might be life-changing, I can only imagine.
If she's letting you fork out $3.7k for a pair of F18s, you're really doing incredibly well on the WAF thing. :-) It's not like F18s are actually small or anything...
Setting your speakers physically on top of your subwoofers is generally not recommended; Dave can give you technical details on this.
Hmm, not sure I agree there. Ported vs. sealed are different, but one isn't necessarily better than the other. Porting is, in some sense, a trick to get more low-frequency response out of a speaker. You get more SPL per watt at around the port frequency, but you get a steeper fall-off below that, so it's more an issue of what you're looking for than one being absolutely better than the other. My HT sub is sealed and it's fine.
Yes, easily. As Dave said, you'd probably be more than fine with a pair of E15HPs. Frankly, a pair of FVX12s would probably even do the trick, especially if you're coming from zero subs. Enrico is a great guy, but his recommendations are generally based on listening at reference-level volume (or higher), which is CRAZY loud. Since you're going even bigger than his recommendation, yes, it'll crush your place.
Hide all pets and small children.