This is not uncommon at all, and is a lot more problematic on the 2007-09 models. With the codes present that you posted, you definitely have a chain issue. The chains in the earlier models are of an inferior design and stretch over time causing the codes you have. There is also a problem with the cylinder heads and the camshafts walking back and forth from the excessive play. Usually when a chain job is done, there is normally a shim or shims put in to take up the slack between the end of the camshaft and head.
This is really a job you'd want a dealership to do. It is very labor intensive and involves many delicate procedures and special tools, so you'd want someone with experience on that car line to do the work.
The price they quoted you is very reasonable, and not at all out of line.3.6L Timing Chains