As you quite correctly point out, natural selection has no meaning until there is reproduction. Reproduction is one of the basic requirements for life, but there are others. I think the other main requirement somewhere along the way would be stabilization of that self replication machinery. Yet we can't have life as too stabile either as then there is no room for higher organized life to arise. Interestingly, life appears to dwell on the edge of this fine balance.
Nobody knows how many self replicating molecules are possible.
DNA happens to be the one all terrestrial life is based on, but it may not be the only molecule that could lead to living organisms. This reminds me an argument IDers employ. So I think before DNA we had to have had some other replication mechanism. There are several answers to this. One is that they probably do arise spontaneously but the environment is now so filled with highly developed living forms that these new arrivals in the contest don't have a chance.