There are a LOT of choices on the market for dash cams, and you get different things with the different ones that are out there. Look for one that has what you want.
Some common things are the ability to simply record over the oldest files when the memory card gets full, shock sensors that will cause an auto-save of the file currently being recorded, ability to press a button to take a snapshot/still photo, ability to press a button to save off the current file in a protected folder, and high quality recording day or night.
Some potential options to look for would be things like a forward AND rearward facing camera on the same device, ability to support very large SD cards (I use 64G cards in mine and get about 6 solid hours of recording time on a card), ability to turn off shock sensors and/or adjust sensitivity, ability to record audio and turn audio off, support for GPS stamping into the file (along with time/date and speed), and overall resolution quality (especially in low light).
When it comes to overwriting old files, you typically need to format the card periodically to be sure that there is available space. My camera will beep at me if there's a recording issue, but I don't think it would be heard over the sound of the tractor engine. My camera records three minutes of video at a time (per file), and I think this is fairly common. It's a good balance between not having a billion files to go through and risking losing video because the file is large and gets corrupted.
I have a Viofo A119 camera that cost me about $100 with the GPS mount. The newer ones are even better than the one I have, and I do like mine. Recording during the day is excellent and recording at night is reasonable. The biggest issue with night recording is that your headlights throw off the exposure for anything not directly in the beam. So, what you can see is reduced quite a bit.