I just want to be clear, if you want to upload to Strava, you need a computer that collects GPS data. The cheapest are the Garmin Edge 200 and the Cateye Stealth 10 (there are also options from Wahoo but I believe they all need to connect to a cell phone- I don't know wahoo well). The Garmin and Cateye units are both solid choices. The Stealth 10 is cheaper, but the Edge 200 has a few extra bells and whistles. (If there's another ~$100 GPS computer out there, feel free to correct me.)

These cheaper gps units don't display your position, they just give you a speedometer, etc., and collect data for later viewing. They're easy to use, just mount them on your handlebar and ride, no need for any extra sensors or magnets on the bike.

If you don't need gps/strava, go with any Cateye computer (Edit: I've also liked Bontrager computers, particularly if you own a newer Trek bike). Don't compromise to save a few dollars, there is a LOT of junk out there. Pick a wireless model, they are a few dollars more, but wires on wired models fail eventually, and it's extra clutter on the bike.