No need to spend money on a big brand, got a cheapo chinese one in the work van that has been running 24/7 apart from weekends for about four years. Its not brilliant…you cannot always make out the number plates but it would prove/disprove fault in a crash.
Got my brother a £40 ebay job and the footage he showed me was also good, went to buy one for my camper and found a £60 one with a rear camera too…it’ll also run permanently and act as a home security camera as it covers where the couriers drop off my parcels 🙂
https://www.amazon.co.uk/gp/product/B01F5C950I/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
Its nice and small, I actually glued it alongside the courtesy light so its not even on the windscreen, very fit and forget.
If you are fitting one due to getting stressed about other drivers, its not gonna help (except perhaps it will serve to calm down your own driving by discouraging you from blocking/tailgating type behaviour). Its only going to be useful when you actually have a prang.