Where are the rest of the parts for that Trek?

A 510 can be worth a decent amount of $$$ if it's in good condition.

But, to answer your question; don't waste any money "upgrading" anything. Just buy a better bike, ride the **** out of it, replace what breaks and be happy.

Unless you're racing, the only upgrades should improve comfort or function. E.g., fenders or a better saddle are good upgrades. A new crankset even though your current one is fine is a bad upgrade.