Not sure what size hotend I have

The bottom line is my brother pieced together a reprap kit, and couldnt put it together. I have the kit fully assembled and working. In pronterface I can heat the hotend, and move all the motors in the correct directions. So I am almost there, but I went to buy filament, and I have no idea what to get. I have a J end with a brass tip. It also has the finned body, so It leads my to believe it is an MK-V or MK-IV nozzle, but I am not sure what size opening it has.

Bottom line, I belive I have a MK-V J head nozzle and am not sure what size filament to get. Either 1.75 or 3, I am not sure. Should I just bite the bullet and buy both?

Can you try to measure the opening at the top of the barrel? the hole should be pretty close to the intended filament width. If you have a 1.75 nozzle, and buy 3mm filament, there's no way its going to fit. If you have a 3mm nozzle and get 1.75mm filament, then you should be able to get away with using it if you turn up the flow rate.