Good to see you are taking proactive steps to improve!
We estimated the maximum amount of weight we were going to lift and then decided how fast we wanted to lift that weight. I think we ended up with a 16:1 transmission with 1 CIM. To insure the transmission had enough strength to handle this load we went with a BaneBots P80 transmission.
http://www.banebots.com/category/P80_CIM_STOCK.html
We also used #25 chain so we could pull the elevator down as we were concerned with the elevator getting jammed.
A nearby team picked a 12.75:1 drive transmission from AndyMark and used 2 CIM motors. If you have 2 single CIM transmissions, you might be able to use both to share the load.
Vex has a nice lookup table to compare ratio to breaking output force.
http://content.vexrobotics.com/vexpr...s-20150106.pdf
Either way you can use bungee cord or surgical tubing to counter balance the weight.
From the drawing, it was hard for me to see how the structure was going to transfer the weight from the middle of the arm to the elevator. Most teams used some angle (diagonal) support instead of relying on the strength of the joint.
For more ideas; find the top teams and go to their website to review their bot pictures or find the top local teams and go visit them.