there's two schools of thought on that. one is that more cores is better, and even an old i7 (my music pc still has an i7-860 in it, 2.93ghz x8, sells for about 100$ now) can be really nice. the other is that ghz rules supreme, and the i5-4790 is certainly a good example of that. look at what you're using, and what you want to use, and scale appropriately. if you're using a 2500K right now and want to double your performance, a 4790 might not be enough - or maybe it will be, depending on whether your apps are properly and consistently multithreaded. i usually get specific examples so i can suss out what's good and not for this.
as for keeping it offline, frankly i just buy the software and then download a crack. that way, i own it, and i use it in the way i want to (which is on a pc that can't access the internet). very few VSTs really don't have any offline activation, though, there's only a few i've had to do that for.