How can I find a GPS unit that can display north at the top at all times?

In most GPS navigator is such a function. But if you are not sure about it, you can look to characteristics or ask some people that have already bought it. I have this function in my omTom Go Comfort 6 and switches on in settings and names "show the north".

The point of having a GPS displaying a map is to know where you are instantly by looking at it, which can't be done if the map rotates. So most GPS'es will have the feature you want, and since it's quite an obvious feature, it won't be listed in the specs.

Simply google "name of your GPS" and "north up", for example "garmin north up". You'll get the manual page explaining how to set it up.

If you want a cycling GPS then the other features you might consider interesting are compass, altitude sensor, and other bicycle interfaces like heart rate monitor or cadence sensor.

Without an onboard magnetic sensor (compass) the GPS has to guess your heading from the direction of your movement on the map, which requires a good enough GPS signal, and it only works when you are moving. In other words, if you're moving slowly or are stopped deep in the woods with bad signal, a real compass works a lot better whether it is inside the GPS or just a good old compass on your handlebar. Also the latter doesn't require batteries and will get you home even if the GPS dies.

The altitude sensor measures elevation more accurately than the GPS does. GPS is pretty accurate for positioning but it requires more visible satellites and good signal for accurate altitude.

And make sure the screen is readable in full sunlight, and the touchscreen still works in rain.

Many even most current models do in fact support this. It's presumably common enough that the manufacturers don't feel the need to specify. Manuals and menus aren't always clear, and experienced owners have told me that it's not an option on their device when it actually is buried somewhere.

Garmin use the term "north up" (as opposed to "track up") and their manuals are searchable online. For some models the setting is a feature of an activity profile rather than global. While my original Etrex lacks this option, most more recent models do.

For Wahoo, disabling "always rotate map" via the android app is the way to configure it, but this only applies if you're not following a route.

In both cases it's a configuration option rather than something easy to toggle from the navigation display. The few modern devices that really don't seem to offer this option are those that that provide only minimal mapping features on the device itself.