I would suggest getting a computer that has current speed/average speed/distance/odometer and cadence meaning that it will have dual sensors. I have a simple wired Cateye Strada and it works well.

When trying to keep an even pace, it is more important to keep an eye on your cadence while maintaining an even effort as speed will naturally vary with the road grade.

Beyond that, if you want to also have GPS tracking with an app, Strava is a popular choice.