I certainly understand about not wanting to upgrade the HU. For me, my hesitation was the quality of look the after market dash kits would provide. I decided to bite the bullet and just upgrade the HU, for me the better quality sound will outweigh the not-so factory look of the dash.
All of this is just our opinion. It is your car after all, and completely up to you on what to do. The Inifinitys will certainly fit in, and sound great in my opinion, it just takes a bit more work.
My main advice: Do it in stages. That's what I'm doing, mainly because I don't have the liquid cash laying around to buy all the components at the same time. But if you're unsure, then do a bit at a time and see what happens and how you like it. You can also go back and return something if it doesn't perform how you expect it.
As for Power Acoustik, I know the brand and I consider them to be very good, especially for the money, like you said. Go for the 6.5 inch subs and let us know what it's like, thats certainly a path that others may consider after your input. As for EQs, I don't know if I've ever seen an EQ with high level inputs, but of course I haven't seen everything!
Good luck and let us know how it goes!