hhey guys ive done some reasearch on buying an audio sytem for my car. i have a few quesstions

I have a 1990 Vn exec commodore

1. can any speakers fit into my car?
2. how hard is installation your self?
3. what is a good amount of watts for an amp to put out?
4. how important is a good headunit>?

thanks heaps for replys take it easy