2000km = 1200 miles. I've never had to replace a chain or cassette at mileage that low. I think the worst I ever managed was 3000 miles (almost 5k km) and only because I used a stupid gear combination too frequently and wore out my 11T cog.

The bike sounds like it needs a tune up and perhaps a new barrel adjuster. The clicking noise when pedaling could be a variety of things but frequently in my experience it's a bottom bracket that needs the threads greased or the cups tightened.

My advice is to let the shop have a go at fixing the issues. If they can't manage it or want to sell you a bunch of new parts either find a new shop or learn how to do your own maintenance. It's not that hard.