Music: Stream from cloud or home computer. There's nothing quite as nice as Simplify Media used to be, but Grooveshark seems nice until they inevitably disable your upload rights due to DMCA takedown notices. Zumocast seems like it might be the best option, but I haven't yet tried it.
Video: Stream from home computer. There's an unofficial Air Video for Mac client in the wild. I found it on these forums.
Documents: Cloud. I like SugarSync, personally, though some go for Dropbox.
Email: Cloud. I think I may forgo any local email client and have Gmail check my other inboxes for me.
Photos: Local. It may be time for me to switch from iPhoto to Picasa for Picasaweb, though. My library is under 40GB though, so I will keep it local for now.
I used to keep iTunes/iPhoto libraries on an external, but you are really, really going to be feeling the speed difference trying to work with hundreds of gigs of media on a spinning HDD. It'll irritate you more than you think. I say go "mini-library" or keep it in the cloud.
Lastly, CleanMyMac has routinely found 7-22 gigabytes of extra space on my drive. Well worth the price.