Unless the nasty customers are well-armed, retail nursery work is pretty safe. It can be rather physical at times—you'll probably be lugging plants around that could weigh upwards of fifty pounds—but it should be quite manageable as far as impact on your body goes.
There are usually lots of pesticides for sale in a nursery, but they're seldom used. The plant turnover rate of a well-run nursery should be high, so there's not much time for a plant to develop a disease or pests that spread to the rest of the nursery. Just don't go ingesting any of those pesticides and you should be fine.