Hey Brasi - first best advice will come from a person who has moved to Jamaica, stayed and is happy and making a living.
My input is given with love and comes from my own research but is definately from a person only going to JA for 8 yrs now.
1. length of stay - 6 mths total per calendar year cummulative as a "visitor"; Immigration will stamp your passport each time you go to JA - if you look close it will tell you how long they grant it for (I have had it stamped 1 mth, 2 mth, 3 mth with no real reason for the difference - usually 3) but no problem you just go to the Police Station before the end date on the stamp and ask for an extension which are usually granted and I think its about $150US now - rules have chgd recentely and you may have to go to Mobay or Sav now but there are lots of threads on this on the Negril board. You can apply for a work visa, and others and the information is all on the US INS website http://www.uscis.gov/portal/site/uscis
WORKING - this is the biggest hold up to living in JA for me. No way to make a living that I have found yet. The few jobs are needed by locals to survive so for me it would not feel right unless I did something that actually made things better for Jamacain people. But there are a few skills that might be needed like architect, medical, teaching, etc.
2. Cost of living - as you said it will depend on how much you are willing to rough it. To me things are very high in JA but its weird some things are less. Cable was about $40 mth; current about same as home depending on usage. Food is the biggest cost in my opinion meaning I spend the most on that - equal to my rent $500US and if you feed others its more of course. I worked 3 jobs for a year and saved up a bank and stayed as long as it would last - 4 mths thats it. Plus putting things in storage and keeping up car ins etc while gone and then $$ to set back up when I got back to US in new apt, deposits, etc. Worth it - hell ya.
3. Owning/Renting - again depends on how long you plan to stay in my opinion and how much you are willing to rough it. I think renting is more flexible but I may buy when I retire but still split my time.
My other advice is to put all this on paper if you are serious. What it will take. Sounds like you are. And talk with Rob/Lisa and people like that face to face with questions. living there is much different than visiting - even jus long term stays which I highly recommend before moving there. Many to learn all the runnings of "life" not just how to have fun on vacation. But if its your dream I think you should move towards the joy.....life is too short not to be really happy.
Hope this helps - again just my take on things - take with a grain of "sand" and BLESS your journey