I'll just say this: my thrill-ride-hating, motion-sickness-prone mom rides both Expedition Everest and Space Mountain, and she loves them both. I actually just finally got her on Expedition Everest for the very first time a few weeks ago, after trying to convince her to try it since it opened in '06. As soon as we got off, she was rushing past me with a big smile on her face, in a hurry to get right back in line! She absolutely loved it and wanted to ride it again immediately.
The backwards portion is about 20 seconds, and as mentioned, it's not really a drop, just some twists and turns. There are plenty of EE ride-through videos on YouTube, so you can time it yourself if you want. And the lift hill looks pretty big when you look at it from the ground, but the hill you see the coaster going up is NOT the size of the hill it actually goes down. The biggest down hill is only 80 feet. That's only 30 feet higher than Splash Mountain, which sounds like a lot, but it's really not.
I won't lie, it's a little more intense than Big Thunder Mountain, but not really by much. As coasters go, EE is pretty mild. Trust me, if my mom can ride it, you can. She'd be the first one to admit that she's a major scaredy-cat when it comes to rides, and she's very prone to motion sickness, but when she finally tried it, she thought it was a blast and couldn't wait to ride it again.
Space Mountain is a piece of cake, very mild. It's basically a wild mouse style coaster, if you've ever ridden one of those. Just a series of sharp turns and fairly small ups and downs. The thing that makes it thrilling is that you can't see what's coming, since it's so dark that you can't even see your hand in front of your face, let alone the track. The turns, dips, and hills come very suddenly, and the darkness makes it seem faster than it is. But all of this just makes it more fun. It's not scary at all.
You can do both of them, no problem, and I know you'll be glad you did! Good luck! :)