Hi Pnina, joomla virtuemart developer here, I am interested in this project.
Like you can see from my reviews I already did this kind of job many times.
All the work will be done on my development server, that mean no downtime for your current live shop, also you will be able to track the progress.
The site will be upgraded with all the data and products in it now, so no, you don't need to enter anything again, everything will stay the same only the joomla core system and virtuemart will be upgraded.
Please send me some more details about the site like payment methods and shipment modules.
Thanks for your time, I am waiting on your feedback!
sincerely
Ivan