They can rust a bit unless you dry them properly, but you can just scrub the rust off with a bit of wire wool. If you're buying, one cast-iron pan (non-enamelled) is the same as any other pretty much, so just buy based on whether you want a square one or a round one, griddle or not. You shouldn't expect to have to pan over £15 for a cast-iron skillet.
Using one is a bit of a labour of love because they're kind of high maintenance in comparison a normal domestic teflon pan. A compromise is an enamelled cast-iron pan ala Le Creuset, but they tend to be pricey and you trade some of the bomb-proof durability of a bare cast-iron pan for ease of cleaning.