Should I be insisting on a certain Google P/R or Alexa ranking for link exchanges? Also, is it true that they have to be relevant? Should I not be exchanging links with sites outside of my business model?
1) Stay relevant 80% of the time. 2) If it's a crap site dont link. 3) pr2+ will count but keep in mind that some pr1's might increase and benefit in the long run. 4) dont pay attention to alexa unless you're paying for links and want traffic from these sites. I'll look at alexa for very general reference only.
Don't focus too much on PR...remember, every PR7, PR8, and PR9 website today started from a PR0. Most quality websites will eventually obtain PR if they continue to develop the site. The most important thing I look for (in fact it's the first thing) is whether or not the content is unique. If the content is duplicated, there is a good chance that the website isn't in it for the long haul. On the other hand, if all the content is unique and well written, you can assume that the website will pick up some PR after the next update.