If you would like to invest in domains, PR doesn't matter as much as you might think, especially with expired domains.
Like I mentioned in another post, you should be looking for keyword domains, ones that you can turn around for a profit, while generating revenue through minisites.
If you'd like more help, feel free to PM me, I'll do what I can to help you out. I've been domaining for more than 3 years now, and I do it full time.