You don't "have to" wear bike shorts at all and there are a lot of people who do lots of biking without them. I do a lot of casual riding wearing athletic underwear and regular shorts or jeans. They do have advantages but don't think "padding", think moisture control and friction reduction. An overly thick chamois can actually be less comfortable than a thinner but well designed one. One of my favorite pair has a quite thin chamois and I've worn them as liner shorts under a variety of cargo or jean shorts for ride of 100-miles or more.

Cycling shorts are like any other pants and selection is a highly individualized choice, but I recommend staying away from the very cheap ones. Aerotech has some decent priced ones that are good quality. I've got one of the Black Pearl and one Top Shelf and find them comfortable and durable. Aero Tech Designs | Men's Road Bike Shorts | Bicycle Ride Comfort The Black P have a shorter inseam so they don't show under regular shorts making them good liners.

As far as the debate over shorts vs. bibs, some people almost reach zealot status in their preference but the reality is that as long as they are a good fit and design, either will serve you well, making it a personal choice. I own both and wear shorts probably 80% of the time.