Sounds like you are doing ok. The plywood t-111 on the gable ends will stop the ends of your long walls from spreading so you can space your three collar ties out at roughly 40" increments. The more weight (snow) you have on the roof the more the spreading forces. Or, if you beef up your ridge beam (double it) and keep a 2x4 or two directly under it to the sill, then the spreading forces will be reduced because the ridge will be holding the roof up more. Your structure sounds fine. The snow load partially depends on what pitch the roof is. The steeper the pitch the better. If you are at 6/12 or 8/12 you will be fine. I prefer 16" on center for rafters with 5/8" ply, but 3/4" at 2' should be ok. When you are at 2' on center you should use sheathing clips to minimize warping problems etc. at long seams. You put them mid way between each rafter between the roof sheathing sheets, see: http://www.strongtie.com/ftp/catalogs/c-2001/page41.pdf They are available at most lumber yards.
I would build up the overhang with a piece of 2x like you propose. You can also hang the sheathing out an inch or two and not suffer visible warping if it is covered with drip edge.