I recommend the RockShox Reverb. It is probably the best seatpost. If you want the seatpost that will operate the best (reliable, not too heavy, convenient), pick this one.
I recommend a 125 mm. I have run into situations where it was good to get. The added weight over a 100 mm is negligible.
Mount: you are correct, you need a RHS to mount it left under the bar. You can't benefit from the MMX advantage though. I have mounted mine in the regular way, using the MMX system (brake + shifter + seatpost in one clamp) and it's really cool.
A dropper post will change your ride. Faster downhill (and safer). You never hesitate to drop, even for something as short as 20 meter if you feel it will help. Also, my top position is now what it should really be (I used to drop a few millimeters to what it should really be, "just in case").