Magento to WordPress 301 redirects

Hi guys,
I have a client that have a Magento site and we will need to move the site to WordPress. He has around 800 products but the new site will have only 300 products.
Questions:
1. Do I need to do 301 redirects to all the products when the site gets transfer to WordPress?
2. If yes, is there any way to redirect all in one instead one by one?
3. Is there any way to tell the search engine, Google and Bing, ignore the old site and see the new one?
4. How can I find out what kind of SEO has been done in the site, like internal links, external links etc..?

The reason why I am asking these questions is because when I searched for “Site: clientsite.com” in Google more than 20 pages of search result came up from that site. And if I don’t handle properly the 301’s lots of 404 errors will come up and Google will penalize me.
Please let me know if there is a better way to handle redirection with a large site so we don’t get penalized by the search engines.

Appreciate your help.
Thank you
EWS

https://wordpress.org/plugins/wordpress-seo/