1.) The difficulty of porting plugins vary. There are, of course, change from Python 2 to Python 3. But you also have to contend with some API changes. You will probably want to take a look at this if you haven't already. sublimetext.com/docs/3/api_reference.html
2.) There are any structural changes between ST2 and ST3 that I know of. Many plugins will work on both ST2 and ST3 with minimal modifications.
3.) I've been using ST3, almost since it's release and it feels pretty stable to me. I think a major limiting factor is the number of plugins that haven't been ported to ST3. I'm sure this isn't a complete list, so you will want to check the actual plugins you use for ST3 compatibility, but it's probably a good starting point. github.com/wbond/sublime_packag ... e-Packages Under the assumption you will eventually switch to ST3, it's probably worth while to get your personal stuff ported over now. If you do decide it's not quite where you would like it to be, you can always go back to ST2. You can install/use ST2 and ST3 side by side, so I don't really see any major downside in at least trying it. I guess you could argue time porting your personal plugins, but like I said, you would probably do that eventually anyways.