After a redesign of the site or homepage, ranking can often drop since google sees a different page. It may also affect ranking in SERP, but does not affect PR as pagerank depends on link factors (mostly offsite), not site content. Also the most important thing is good SEO, as long as you practice it right on your new design, your site will eventually come back to its place and possibly better Also bunch of your visitors might not like the new look and choose to leave at first. I have done some minor changes to my site multiple times and I never saw any change in PR. However, changing template etc can affect the position of site in SERPs. Also make sure you have all your in-bound links in tact. Remember, the true purpose of SEO is not to have a high Pagerank or even rank well in the search engines for some keywords - instead it's to use the search engines to drive as much qualified targeted traffic to your Web site which should be done through great rankings and content.
Page design will not affect your PR. You can have a top notch design or a blank page but PR will stay the same. Links going out doesn't do anything about your page PR either. So you can have 1 or 1000 links (out) and your PR will stay the same. Also a do follow or no follow links to other pages makes no difference either.
I guess no-follow would be enough for new links. Hope you will change the design only, not gonna change the present url structures. content, and on-page factors like page-title, h1, h2....