So I'm trying to work out how best to proceed at this point.

My current setup is basically a work folder on my desktop PC with a subfolder for each client containing:
-A master excel spreadsheet with their contact info/websites/targetted keywords/passwords/web hostings account/etc
-A folder for anything web design related
-A folder for anything SEO related

Digging down past that folders and structure becomes much less uniform as it was really just done on a case by case basis.

The advantages of this structure is I have the synced copy of the folder on my backed up NAS so it is pretty secure.
The disadvantages are starting to show through now though as my customer base grows it is becomming less organised/structure.

And the main problem: It isn't mobile, I'm starting to be at my desktop PC less and less and therefore would like a hosted system to manage this.

What would you recommend to manage client information and documents?

I use Google Docs

If you have a smartphone and want to keep the folder system you have now, I recommend using something called Dropbox. It's pretty much a cloud drive that syncs with almost any device, you get 2GB if you sign up for a free account.

I have a dropbox account on my home computer, my laptop and my iPhone. To read my documents on my iPhone I just downloaded their iPhone app and downloaded a document viewer. Voila! You can view your documents anywhere, anytime. The other great thing about Dropbox is that it will save locally on to your computer, so if you don't have internet, that's okay, everything will be on your harddrive and will re-sync when you have an internet connection.

www.dropbox.com

Dropbox,evernote or even use the quality control wordpress theme.