Hi guys, first time poster here. I'm hoping since this community is UK bike specific I can find some guidance.

Just today I went and purchased a used Apollo Ridge All Terrain Mountain Bike. It's in pretty bad shape, but hey I paid only £20 on it. I did consult a repair shop and he told me about the repairs it will probably need (he suggested it's going to cost all in all £60 to £100).

The bike still functions at the basic level. Breaks work fine and it runs. I'm thinking I'll just clean it up and maybe only replace the chain and gears if need be. See how it works from then.

If I had 100s of quid to spend I wouldn't have purchased this poor wasted soul in the first place.

The main question I have is, can someone at least point me in the right direction by telling me what I need to learn about chains and gears to decide whether to just clean it or replace the chain gears?

If replacement is the right way then what are my next steps?

*Never had a bike before or done any work on it. But I'm determined to learn.