General Question about starting a website project

I am starting a large WPMU site that may have many users in the future. Here are the things that I think I need to do. Is this the correct order/is there anything missing?

1. Install all Themes/Plugins needed

2. Test out plugin/theme functions and make sure they work (like MU)

3. Edit theme for a nice UI (usually takes the longest time, at least for me). I will try and use a child theme for this (to ensure future updates do not hurt the site)

4. Make minor tweaks in display (branding, etc).

Your thoughts / input is greatly appreciated.

Thanks!