Regarding Blog

we do not have a blog on current site, however we do have a blog on our old site. Can we link to that blog, or will different server and web address cause issues with SEO. Please advise what is the best way to handle the blog

What?

If you link to your old blog, you will be passing link juice from your new site and this is never good. You have two options:

1. Copy/paste the posts from the old blog site to a new blog in the new site and put canonical metatags in the old posts that point to the new posts
2. Just migrate the whole blog to the new site and do 301 from each blog post to the new blog posts. If they have the same slug you can do one 301 redirection with the adequate regex formula.

Check this:
https://moz.com/blog/301-redirect-or-relcanonical-which-one-should-you-use