The first three movie DVDs are actually very easy to find where I live. I haven't seen movie 2 as much as the rest, but they're all in pretty good supply. Mewtwo Returns isn't too hard to find either, at least for me.
If I were you, though, I'd keep my VHS's until I bought all the DVDs, just in case you can't find a movie.
I tell ya, though, buying the DVDs for the early movies has always been bittersweet for me. On the bright side, you get commentaries, which are always good, but the bad news is that the movies are always in pan-&-scan.