+2 on not using flat! I never use flat paint for anything personally, the only time I use anything with a "flat" finish is a primer.
I'd go with a semi-flat if you don't want the shine and still want a paint that's not as porous.
If you don't mind the shine then gloss or semi-gloss is the way to go.
I always use primer on a bare wall. You could use a 2 in 1 paint & primer, less work that way..
If the old paint is not peeling and there aren't too many layers underneath it to begin with than you can clean, prep and paint over the exsisting paint. That's not always the case though..