What do you look for in a forklift?

Any forklift with hard rubber tires will not fair well on gravel or dirt surfaces, nor will one with small diameter tires. I work with and around them every day and I have seen dozens of different machines over the years. If you expect to be able to take it off pavement it needs to have pneumatic tires, at least 16 inches in diameter and with some sort of tread. Rear tires should also have tread. It really depends on how soft the surface you will be operating on. As for electric or propane, they both work well, but price may be the deciding factor.
We do take our hard tired forklifts outside on the gravel, but as soon as we get away from the really hard packed areas, they get stuck easily.