I would have to say that the half dollar album is by far one of the easiest albums to fill today when starting from scratch/zero. GO FOR IT. This is especially true if you buy half dollar boxes even just once in a huge while. Plus, you can just keep replacing the average ones with newer, shiny BU halves as you find them (upgrading). I would buy just the regular book that does NOT include the proofs; you will find proofs in $500 boxes on occasion, but not enough to warrant buying the proofs book.
As NHsorter mentioned above, the 1970 is the key coin and is the hardest to find (although I think CardsnCoins found one once from circulation). Most folks at coin shows will sell it to you for a premium but not a huge premium. The other coins that are only slightly hard to find are the 1986/1987 halves; still, I have found plenty of them in circulation (I only save them if they are BU). So in my opinion, the 1970 silver half is the only one that you will have to seek out manually from a dealer at a coin show; just trade it for a 1965-1969 silver half and a little extra!