does the 2004 Sonata use a timing chain or a belt and at what point is it recommend to be changed

The Hyundai Sonata manufacturer recommends replacing the timing belt every 60.000 miles or 4 years whichever comes first. If you want to go over this recommendation, you might be able to pull more miles out of the original timing belt. I wouldn't chance it. The engine in the Sonata is an "interference" engine. This means that if the timing belt breaks, the valves will hit the pistons and be bent. If the valves are bent usually a new or remanufactured cylinder head is required. Changing the cylinder head is expensive. ALways do the correct maintenance as specified by the manufacturer.

What can I expect it to cost to get this work done and should I use the agents or can this be done by any competent mechanic.