The most important word of advise?

Listen for yourself and make up your own mind. You will eventually find what matters to you and you can look for the components to fill your dream system. All stereos are a compromise and you have to find what suits you.

I couldn't limit the advice to just one item of importance. However, I will limit it to just several of the most important.

1. Start with being reasonable about what the size of your room will accommodate and choose speakers accordingly. Those big floor speakers may sound great, but not in that tiny, converted bedroom system. Only if you quickly intend to use them in a larger room should you consider floor models in a small room. Stand mount speakers will almost always get you closer to the music in small rooms.

2. Speakers and amplifiers should be chosen to be compatible with each other. A technical mistake in compatibility is a mistake that can only be undone by selling one or the other, usually at a substantial loss.

3. Proper speaker positioning is the single most important starting point and will effect the frequency response more than any other factor. Read and learn and know where they are going to go before you even audition.

Music is more important than equiptment.

Buy stuff you can resell easily if it doesn't work out.

start with loudspeakers.