Best practices to setting up a test site for testing design ideas different themes, customizing code

Hello, I thought that I had asked this before but can’t find any post about it.

What I want to do is set up a site to help me test out different theme’s, designs, plugins and learnin to code and /or customize sites. I may eventually want to learn about multi sites etc… but currently want to learn and test ideas without using my live site and losing sales.

So I’m not sure if it’s better to set up a developers type server and if so should that be on a local computer, or is it better to set up a new domain and work online live.

In the future (if business picks up) I hope to add more clients that I will be maintaining their online presence and would like a convenient way to be able to do that. Including setting up different themes so they can approve the design they like the best.

Any and all help is appreciated.

My current task (is also in another thread) is trying to set up an online store via marketpress and having password protected pages so only certain clients can access only thier content. I had been experimenting around on one of my live sites with a hidden page but soon discovered that my real items for sale became unavailable.