Car-savvy jellies: what's the optimal $ cutoff for buying a used car?

Someone who I thought was pretty knowledgeable told me this years ago, when I was shopping:

If you can’t spend at least $5000 on a used car, buy the cheapest one you can find and spend what it takes to fix it up.

Is this good advice? And what would that $5k figure be now?

Thanks.

        —Completely Unknowledgeable