1. Rangefinder, do you need one?
What good is a rifle that is capable of shooting longrange if you don't know how far the target is?
2. Are the cheap one good enough?
My first rangefinder was a Bushnell 800. It did a decent job. Inevitably though, you want to range and shoot things that are farther and farther away. As a result, buy the best and be done with it (buy once, cry once). Personally, I have a Swarovski and like it a lot. Although they are $900(US) new, they can be found used for $700ish. Don't fear buying a used unit. I bought mine used and two buddies bought their's used too. They all work just fine. If there was anything wrong with them, the better manufacturers have an unconditional warranty on their units and will repair/replace the unit for you.