As for that dash kit, good god is that thing fugly. If I could, I'd have a puking emoticon but none of them are loading at the moment.
You can get the OEM dash kit that'll house an aftermarket unit. Riverside Infiniti in CA has an eBay store and they sell these kits as they have them, but they are always in limited quantities. You can also check out
http://www.grubbsperformance.com as well.
No stock amp with a sub of any kind.
Oh, and also, I would never ever buy from a company that spells Infiniti as Infinity. Never. If they can't get that right, who knows what else they can't get right.