a) Depends on how much you pay for it? How many OBL you expect to have. If you run a network of sites, you can split the cost, even sell links
b) PR is overrated but still gives you some sense of authority.
c) Godaddy has some auctions, you can get some cheap ones from overseas forums also (DP,etc). Be careful of fake PR and you do have to do maintenance to keep those PRs up especially for dropped domains.
