The problem might be that you have to have Lion (10.7) installed to upgrade to Mountain Lion (10.8) which is a pain. I don't think you can go straight from Snow Leopard (10.6) to ML. Someone may correct that.
Correct. Also bear in mind that the outgoing OS X 10.7 needs to be fully patched up too... you can't just bung in a 10.7 disk and then update to 10.8. It needs to be patched up to the latest 10.7.(x) iteration.
May be less hassle to jut backup files and go for a clean install instead of an upgrade (well, you should have a backup routine in place already anyway !).
I think there was a reversal of screen scrolling direction in one of those updates, to match the way iOS devices scroll, which was one of those that you had to get used to. (Now I wonder how I coped without it).
You can change this back to the old settings easily in System Preferences.