Hi. If this site replaces one that was already at this domain, you should map the old page urls that were used on the old site to an equivalent page on your new site to prevent 404 page not found errors and allow search engines to follow and retain rankings for pages that are already indexed.

You are best off having different pages. Google still reads your site/homepage as 1 chunk of text. So keyword density will be lower if you focus on specific parts. If you split this up into pages, it's more likely to have users coming from search engines. This because pages will have a higher density for the specific keywords you used. Also, your URL will match these keywords (think of Project Management which will have the /project-management URL) along with the H1 tag.

One pagers are awesome, but not if you want to combine that with SEO.