This forum is a good place to ask a specific question, when you know what the question is! You will not have a problem with your iMac that others have not already experienced.

Comprehensive hardware repair manuals were kept secret by Apple and their network of licensed repairers. Having said that, most specific problems can be identified and fixed if described. You can find good take-apart guides online.  Most parts are also replaceable except probably the CRT itself. At some point you will have to decide how much time and/or expense you are willing to put into maintaining your iMac.

There is a high probability that many problems you start to notice will arise from failing electrolytic capacitors in machines of iMac vintage. There are many threads on this forum about caps replacement.

Rick

Rick