I'm after a backup solution which can:

a) back up only selected folders (excludes Back Blaze?)
b) runs at a scheduled time and not in the background (excludes Crashplan?)
c) backs up to external hard drive
d) only syncs changed documents i.e. doesn't have to back up whole folder again if nothing has changed
e) (Optional not a dealbreaker) Can back up to a cloud location such as Dropbox

I'll have 100-250 gb to start with

Any recommendations? I'd rather not spend $50 on software if possible.

I'm a bit confused by the 'exclude' functionality to back up selected folders on Time Machine - is it easy to just back up your documents folder? It looks to me like you need to exclude all other folder...

Thanks