Of course it's possible to single crochet an afghan, but it would take a really long time. Try double or treble stitching every other row, to make it go much faster and have a nicer texture.
I've done several single crochet afghans and they have been very popular. Because it is a basic stitch the whole way, I've found them to be quick because I don't have to thik about what i'm doing. Go for it!
Of course it is fine to crochet the entire afghan in single stitch. There are actually many designs that will call for single crochet and it will be warm.
I'm sure it's okay to do this but it could be very time consuming. You may want to look online for ideas and techniques on knitting to learn more. There should be a lot of good resources available to show you what you need to do to get started.
