For my senior project we made a man powered water distiller. Our requirements were that we couldn't use a filter, or an external power source (fire, batteries, sun, etc). We actually made one that worked, and learned a whole lot.

Anyways, you have to prioritize. What is more important to you, and what is your budget? If you are good at working with metal, then make one yourself. It is pretty easy. I don't think there is an off the shelf unit that uses a fire (I haven't looked). An RO filter is expensive, but is also small, light, and you can get it off the shelf.