Help me with buying a cycle

Hey all. Basically I was going through all the posts and couldn't find one, which would do me. I'm thinking to get a mountain bike just for occasional/kind of sport use, but I like reliable things, which doesn't brake after first time. Could you help me to find out more about cycles and cycling:

1. I know there are different sizes of cycles and wheels as well. Say if I'm 176cm height what cycle I should be looking for?

2. Also, what are good brands of cycles, while I don't want to spend too much on it? Maybe someone could advice me on the choice of cycles in around 500 euro. Would it be possible to buy a decent brand new cycle or would you advice to go for a second hand one.

3. If somebody has a nice bike for sale, please let me know, I might take it . Send some pictures as well.

4. One last thing, please advice on cycle websites and shops, which give good value for money.

Thanks in advance. This will be useful for me and many others.