Hi,
My name is Huy. I have worked with Joomla for 4 years. As you can see in my profile, I have upgraded many Joomla sites. All your content, data, links, modules, menus and template will be kept.
About SEO, your URL, meta title, description will be kept so it doesn't affect to your current rank.
About database, I couldn't access to [login to view URL] It shows 403 error so I couldn't check.
Overall, I can finish this project in 4-5 days. Please feel free to contact me so we can discuss more.