Don't buy ones with backsights on unless you are doing them, as it only gives people the chance to use the wrong scale. For humid areas, get ones with ports for flushing. Suunto's can be bought cheapest in UK from BCRA Cave Surveying Group . 30m is good. Small ones tend to fill with mud.
Open reels break.
All tapes break or numbers rub off. Very hard to do well until you have tried to use somone else's info to draw up. The idea is to leap frog along the cave measuring the direction, derivation and length of the legs, while recording the dimensions and sketch of the passage. However the amount of information collectable is infinite and time is usually short, so set out here is what is considered the essential information and a few ways to collect it. Teams of two, three or four are used.