Here's my take:
1) The height of the chimney on a pellet smoker doesn't really matter, for one simple reason: The smoke is not drawn out, rather it's forced out by the induction fan. Some pellet smokers do not even have a chimney for this very reason.
2) Smokeasaurus nailed this one:
The downdraft mod is well worth the coin. It not only channels the smoke along the grate level it also helps with evening out temps from one side to another.
Again, keep in mind a Pellet Smoker uses an induction fan, which forces air/smoke through the chamber. The downdraft baffle helps direct and keep the smoke in contact with your food longer!
One Caution: Be absolutely sure that the downdraft baffle you buy is designed for your grill so that the air flow rate does not change. Restricting the airflow will definitely cause problems!
3) I agree with the others. The Maverick ET-732 is a GREAT unit. I have one that is 5+ years old and still works great! As long as you are careful you can run the wire under the lid without trouble. Two things to avoid: a) avoid kinks in the wire, and b) avoid getting moisture into the probe where the wire goes in.
4) Your call. The Traeger grill/griddle may be designed to fit your grill better.
5) Savannah Stoker is a forum sponsor that makes and sells PID controllers. They are probably the best after-market controllers out.
Here: a link:
Savannah Stoker6) Rib Rack versus Second shelf? Again, personal preference, but if I had to choose only one, I would go with a second shelf because you can do more things than just ribs with a second shelf.
That's my 2ยข and that's probably what it's worth, but I'm sticking with it!