Purchasing all instruments doesn't work in GarageBand 10.0.2
Apple have made all the instruments free to download. It appears older versions of GarageBand don't understand the server response after this came into effect, and therefore it's not possible to download them without updating to the latest version of GarageBand.

It may be possible to use another computer or virtual machine with a newer version of GarageBand to download them, then copy them to your own computer, but I haven't tested this.