iPhone 4S w/ iOS 6, any risks in finally upgrading to 8.4?
To answer your questions in order:
1. No, you can only update to 8.4.
2. I don't have any problem composing emails with 8.4. I believe that there is an Accessibility setting for font size, anyways.
3. YMMV. My iPhone 4S ran great on iOS 8, but some don't have that great of luck.
4. I think 2.5GB is enough to download.
5. I believe any connectivity issues with iOS 8 were fixed within the first few updates.

iOS 8.4 ran well on my iPhone 4S, but when I gave it to my Mom, when I upgraded to an iPhone 5S, I've noticed that for whatever reason it doesn't work as smoothly as when I had it. Also iOS 9 is right around the corner in the fall, likely next month, stability and performance are what iOS 9 is mainly addressing and the iPhone 4S will get that update, so you might just wait for that.

I haven't really ever had any major problems with iOS 8, I think it is pretty safe to download.