OT: Mac OSX upgrade and general housekeeping

I was asked to help with 'no space on startup disk' issue, only to discover it's a Mac, I have no experience with Mac,

df showed 100% utilization on built in 500gb hard drive, 
I've transferred some user data to external drive, now have 97% 

What's a minimum hard disk free space one should maintain on a Mac?

This runs OSX 10 7 5,  to upgrade to Yosemite, is it just 'click and watch'? 
How much free space should I make before attempting upgrade?

Are there any good housekeeping utilities to use, going through some apps, I've found a photo app with over 1 gb of deleted photos.
(something like ccleaner ?) Any hard disk area to check for junk, like temp in windoze?

Thanks for any pointers

 Voytek
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