I'm wondering whether you would want to consider a mid-drive motor instead, if pursuing an ebike build. 750 W delivered at the hub, without the benefit of gearing, may only be marginal to attain your envisioned speed. But in a mid-drive configuration, 750 W should hold 50-60 kph without an issue. The reason for going higher power beyond that would be to improve the acceleration at those higher speeds, but if you're planning for mostly back roads, then you won't have the stop-and-go or slow-then-go scenarios that might warrant more power.

To reduce the power consumed, also make sure to consider the tires. Certainly, aerodynamic drag starts taking effect after 20 kph, but the rolling resistance of some of the fatbike or dirt bike style tires mounted on some ebikes can be serious sources of drag as well, when running at higher speeds.

In my case, my street-running ebike has too wide of tires, being 26x4.0, and I don't have the option to go narrower because of the rim width. In future, I plan to build custom wheels that has a 220mm wide hub but a 45mm rim. This will look ridiculous, but it'll let me take narrower tires that run higher pressures and have lower weight penalty from the anti-puncture layers. If you're going to take an existing frame, pick one with narrower hub widths, tailored to the surface types you intend to ride on.

My "test" for rolling resistance is the speed I can attain with motor off, pedaling only at a leisurely pace. With my acoustic bike, 20 kph is easy with no heavy breathing or anything like that. With my ebike, it's more like 10-12 kph. That doesn't sound like a lot, but it has huge consequences at speed. Anything to get that pedal-only speed up will pay dividends at the top speed.