M4300-8X8F stack - firmware upgrade question

My experiance is not that high, please bear with me.

We operate two M4300-8X8F in a stack configuration untouched since a couple of years.

As I need to add a new stack member (also M4300-8X8F), it is probably a good time to upgrade the firmware. At the end, all switches must have teh same firmware version, right? I do not have a reserve hardware. 

The currently running firmware is 12.0.11.8 (active image2 on both stack members)

The latest available firmware is 12.0.17.10

Questions:

Is it possible to upgrade directly from 12.0.11.8 to 12.0.18.10?

Should I upgrade before extending the stack with unit 3, or afterwards?

Are there specifics I need to take care of? What is the best way to proceed?

Is there a chance that the configuration will be messed up?

What are the ways to minimize risc?

I guess I can copy the current version 12.0.18.10 via HTTP File Upgrade to image1 on both stacked devices, and the third unit too. Than change the next active image to image 1. A major question is probably if joining the stack before or after the upgrad is better.

Thanks

Dan