I was wondering if a mossberg 835 camo thumbhole stock with a 20" barrel on it is a good buy for $299.99. The only thing im worried about is the barrel length. Is it long enough?

Yes it is long enough....some would say a perfect size for a turkey gun.
If you are planning on shooting heavier than lead loads you will need a new choke tube since the factory one is not rated for it. If you are shooting lead you will be fine. I have gotten excellent results with an Indian Creek, and Hevi-13 3" 2oz #6, or Hevi-13 3.5" 2.25oz #6.

Not sure what the length is on my 835 off hand but you really don't want or need a long barrel on a turkey gun.With the turkey choke in there along with #4 turkey loads all the birds I've shot died quite suddenly.

The length of shotgun barrels can be considerably shorter than they normally are and still get optimum velocity. The idea of needing an long barrel so it would pattern tighter or "shoot harder" was valid back in blackpowder days, and has been myth since.

I have found that a comp-n-choke xxxfull .670 will do the trick along with win x-tended range or high velocity 3.5 # 6's put them down at 50 plus yards. I would never reccommend shooting that far but the pattern you'll get from this setup at 50 yards is more than enough for a clean kill.