Nothing.
In fact, you get nothing for clearing all challenges, and not using a single golden hammer.
In another thread(on these boards, forgot what it was called), someone confirmed it and said nothing.
Well, maybe you get a notice(the person just said nothing). Also, nothing for getting 100% also on SSE also.
Also, there are no unbreakable stuff on the PAL version of Brawl, according to the super-smash wiki on the differences between the Brawl versions.
Personally, I'd use the hammers on things that I'd have to actually "try" to get to get.
For things I don't want to use hammer on:
1. 100 Hours of "vs" Brawl. Eventually, I'll get this anyway. Unless you play ONLINE only(which doesn't count for challenges, unfortunately), youe will get that eventually.
2. Stickers. Unless you have items off all the time, you'll eventually get all stickers. Otherwise, just HAMMER IT if you want to but if you play with items, just wait it out.
3. Trophies. Unless you're completely ignoring Coin Launcher, this is also one of those that you'll probably get by just playing normally.
I usually do a coin launcher every so often.