Sorry your post is a little confusing, it says "and after riding a full suspension yesterday 6 miles killed my butt ".
Does your MTB have full suspension or is it just a hard tail.
A full suspension bike will be far superior to a suspension seat post, I am speaking from experience.
Most seat posts come in 1 of 3 diameter sizes - 27.2mm, 30.9mm and 31.6mm.
You will have to find the diameter of your seat post tube first, maybe if you post the exact make and model of your bike we can help.
However I would imagine swapping the seat out only will make a big difference, but getting the correct one for your "butt" will be a bit trial and error.
As for drinks bottles etc, again it will depend on your bike model and frame, so a model will help. Most are fixed with two screws set a standard distance apart, your frame will have a couple of possible locations, but it will also depend on what else you will want to carry, and how much space you have for what size of bottle etc.