Normally thematically relevant links are rated higher than non-thematic links. I would say that it is better to have a few low PR links that are thematically relevant than have a lot of links that have a completely different theme to your website. People who buy lots of high PR links that are not thematically relevant should be avoided as they could be involved with Link Farms. Google frowns upon Link Farms and similar and will punish anyone who they find is involved with them. Hope this helps
I think quality will usually out weigh quantity, a few high PR from highly related sources will do wonders.
THis will depend on what result you want, do you want to drive traffic to your site? or do you want to have backlinks? posting to high pr and relevant sites can give you traffic well even you post it to pr n/a sites as long as it is relevant it can drive traffic to your sites, gaining backlinks is more on posting to high pr sites and do follow well you can also post to no follow sites as long as they are authority sites