Deployment Help

I’m about to begin building a WordPress website for a church.
I plan on using MAMP to build the site, customize the theme, and import media and content on my own computer. However, the last time I built a site this way, it was difficult to import all the media seamlessly.

Do you have any tips to help me make this deployment more seamless? The church’s current website gets a lot of traffic, and we would prefer to not have any downtime.