My guess is computer hardware and a source of electricity, so he can finally get started on one of those big projects to gain large amounts of personal power / resources.

The economy-disrupting arbitrage scheme was introduced as early as chapter 4. Lots of money would have helped Harry in pretty much every arc. He could think of many, many other schemes to help him finally start world optimisation in earnest. But he never did that. In story logic, he never got round to doing it due to interference from all sides. In writing logic, this can only happen in the final arc because otherwise, the story will be known as "Harry Potter gets really rich" and cease to be about the methods of rationality anymore.

That doesn't necessarily point to computer hardware, but chapter 87 mentions them as a powerful tool that wizards lack ("If there's a money-making method in this book that sounds difficult for a wizard, but it's easy if we can use Dad's old Mac Plus, then we'd have a plan.") and I have a hard time thinking about world optimisation plans that wouldn't benefit from at least some sensible organisation of data, scheduling and on-the-fly printing.