Your amp will run MUCH easier and more efficient at the higher impedence load, but yeah, 400W ish. (actually super low impedence is what's hard on your equipment).
FS is free air resonance, has little or no relation to how low the sub plays, it's used with other parameters to predict the sub's performance.
Again, the second 18 looks OK, has a decent sized motor, sturdy looking basket and a 4" coil. IF you have the 100$ to gamble try it out then if you have a solid box for a single 18 build you can grab something different and compare.
I'd say pyle makes bad stuff, but they really don't make anything, they just slap their name on whatever they can get really cheap from China. The problem with those type of brands is that most of the stuff they sell with their name on it is total garbage. You may find a decent quality product from them now and then, but odds are better than not that you'll be disappointed.