1. Your weight would not require heavier duty shock cord, I use 3/16" cord for that application. The length required depends on your set-up, I would plan on 10-12' of cord. Its not a bad idea to have a couple spools of shock cord around, for me it wears out quickly and everything works SO MUCH better with good cord on it.

2. You can purchase sail cloth repair tapes that would be much better than duct tape for these repairs. A sail repair shop could sell you some patches, or you could purchase a roll online for a few bucks. This would be a permanent repair whereas duct tape will fall off after a short time and leave you with a mess of adhesive.

http://www.sailrite.com/Insignia#!Insignia