My advice is to upgrade only when parts actually wear out. The money you save can be applied to the new bike, and you're already well set up to test the waters with the gearing. Parts, purchased individually are never as economical as they are when attached to a full bike.

The main factor contributing to durability will be your initial assessment of the condition of the parts, by re-packing and adjusting the bearings, so you know that they are maintained according to your standards.