If one wishes to add in a sub, would one then need a 5 channel amp ?
You can get a mono (single channel) amp to run the sub, but it would probably drown out your speakers if they were only hooked up to a HU. I have a 5 channel amp, but that extra channel makes it more expensive. What I like about it is it makes the install/setup easier and cleaner.
As for speakers that are for just the HU, wattage is the key specification. 50-60w would be my preference, sensitivity probably won't be noticeable. The big names will all be close to one another. What really counts is which sounds best for the money. I'd check some speakers out before I purchased, although the big names probably all sound pretty good.
3 way or 2 way? If they are only for the HU save your cash & purchase 2 way speakers. I'd lean towards 6x9 for that application.
I have high power 2 way 5x7s on my system, they perform well in the mid & high range, while the sub covers the lows.