1. Agree wholeheartedly with Stevej24 about the one link. Unless there is an option for a social link or something. The best answer to this is to see what other posts are doing. Remember that each website is its own community so to get the best traction comment wise or with guest posting is to see what's already there and mold yourself
3. Again, this depends on the community of the site. For example, a small site with a strong following might be a PR1 but you'll find tons of quality traffic. Whereas a PR 5 or a huge DA site might just send you spammers. In general, a thoughtful mix is always wise.
4. The only negative I've had is a lack of response, which is pretty common if you submit to huge sites. Be sure to read everything you can about guest posting on a site. For example, hbr.org tells you flat out not to expect a response before 4 weeks. If you have a time sensitive post (which you should) you are better off going to a site that is going to welcome you with open arms
BTW, as a real estate person are you interacting on ActiveRain? We write for real estate and have only heard great things about that community. I keep trying to get us on there for interaction but it is too far down my to do list