What really matters is the height and length (other factors become important when you start getting larger).
What are the dimensions?
Depth effects the pressures that will be seen at the seams and on the glass.
Length matters due to bowing from those pressures.
As far as silicone goes, the Momentive 100 series has really high holding strength, as does SCS 1200, and another that is escaping me right now. I'll have to see if I can find that one as it is specifically manufactured for aquariums (not the stuff you get at the pet shop, that stuff is extremely weak and is probably too old to be useful). Dow also makes one that is good. People use the stuff from the big box stores, but I've looked up the data sheets and they really don't have good bonding strength or tear strength and the shear strength is abysmal. It isn't designed for structural adhesion, it is for sealing joints to make them liquid tight. That and if you get the wrong stuff you can wipe out your tank from inhibitors.
The Momentive 103 is black, 108 is clear, I believe 106 is red and there is a blue as well as white.
As far as the base goes, put a neoprene mat down, or go with a yoga mat or foam board. It will even out the pressures on the bottom glass and prevent (small) imperfections in the surface of your stand from blowing out the glass. You will need a flat level surface for your stand to completely support the bottom of the tank, you can't build it to support only the perimeter. Unless you build it with a raised bottom, in that case you would need a trim of some sort that would help support the seam along the bottom glass like the mass produced tanks do as the seam will be seeing high shear stresses, which is the weakest type of joint for silicone.
I usually feel kind of guilty using the quick reply... my replies are rarely quick.