I remember when a 20 watt car amp was considered quite healthy. Of course that was 30 years ago.AACK!
He's right about deck power- depending on speaker sensitivity, a stock deck can sound good, it just wont be loud. I feel this is an exception to my "front stage only" prefrence. If you only have deck power you're gonna need all 4 speakers running for volume reinforcement. I've read that a decent stock deck is equivilent to a mid-line aftermarket. There was a guy on a car audio forum I used to frequent who built a competition winning system using a stock deck and a small 20 watt amp on eack channel. It wasn't loud, but played within it's limits the sq was fantastic.
If you get component speakers they will come with crossovers which will need to be mounted somewhere dry.