Well, the amp question depends on what you are looking for. As others have stated, the Infinitys will sound better with a good external amp powering them. The stock will do, but a dedicated amp can mean the difference between "sounds good" and "sounds awesome."
As far as installing a sub later on, you have options here too depending on which route you prefer to take. The Kenwood wOOx is self-powered, but rated at only 60 watts. However, it's still a pretty sweet sounding sub, outputting tight bass that doesn't sound boomy and won't drown your mids and highs. And since you might be able to install it under the seat, you'll "feel" the bass just fine.
If you are looking for authorative bass, you will need a set up in your cargo area with nice custom enclosures and a dedicated amp for the sub(s).
Alternatively, you can do what I did in my last car. I had the head unit drive the speakers and installed a stand-alone amp for the bazooka sub. I was running Diamond Audio speakers and found that the HU supplied plenty of juice to drive the speakers. However, I did toy with the idea of installing a 4 channel amp that I was planning on splitting between my rear 6x9s and the sub. I was going to bridge the HU to power the front 6.5 and tweeters. Unfortunately, they broke in to my car and stole everything...so I never go that far.