Sean;
Bill Hawes WShell has filename completion (and a heck of a lot more).
WShell has been around for many years and is one of the best, most solid
products for the Amiga of any kind. It is a commercial product. I have
been using it on my machines now for years and would be about lost without
it.
As far as diskcache goes there are a couple. The Amiga itself does some
diskcaching and adjusting the number of buffers with AddBuffers may do
what you want accomplished.
-bill