You really don't need to spend much on a monopod, they're nothing like tripods in that respect - £30-40 max. Spending a lot more on a carbon jobbie is a waste of money IMHO as the difference in weight is tiny.
Consider the height, and if you go birding or shoot aircraft, you'll need one that can put the camera above your head. You can do without a head very well if your the main use is for maybe panning sport/action with the camera horizontal, or are just using the monopod to take the weight of a heavy lens. But for shooting at an angle up or down, and for switching to vertical framing, you'll need some way of doing that. Some people like ball-heads for this, others go for a custom monopod head.