Do I need an expensive bike / wetsuit?
The most important thing with a bike is that it fits properly and is roadworthy. An entry level road bike is fine or a second hand bike if you can get one. It's advisable to get it checked over by a qualified bike mechanic before you train / race on it. You can hire wetsuits and often then buy them at the end of the season if you want to.
Are triathlons expensive?
Some events can be a bit pricy so try to look for companies that offer free race vouchers as payment for marshalling. This is also a great way to learn more about triathlon.