If you haven't ridden in a long time, you probably won't like a bike with drops, but maybe you will. The "safe" recommendation is a hybrid. Giant Escape, Trek FX, Specialized Cirrus.
Avoid suspensions, fat seats with shocks, knobby tires (spending $ to replace the tires right away eats up your savings right there), etc.
If possible, bring a knowledgable friend along to help avoid getting a clunker. Walt's advice is excellent. See what you might like BEFORE you go. I'm guessing this sale is probably a mob scene.
Helmets are all about what fits. I would try on a few in person. Generally, u-locks are the most durable locks, but a determined thief can defeat any lock given 5-10 minutes (or less) and bystander apathy. My bike isn't super spendy and it often has a kiddie trailer attached for extra deterrent. I use a cheap cable lock personally.