
Altura are good budget panniers, but check out Karrimor too, they have a great reputation. Avoid any pannier that uses elastic bungees to secure to the rack, they have a nasty habit of getting tangled up in your rear wheel (speaking from painful experience here).
Ortlieb are by far the best, but they are outside your budget. imo they are worth it as they last so long and are so tough (and light).
There is no simple way of attaching panniers securely, although some people use those very lightweight cable locks you can get in camping shops. Most thieves are too thick to know how to operate the release clips, so they just try to steal whats inside.