To answer your 1st question, 50% will be about it (If you find someone in the mood to spend)
As far as shops doing poor work;
The more shops I deal with the more projects I take on and learn to do myself. The last time I took a car to a chassis shop I very politely said
"I am a firm beliver in a fair exchange, You want to be paid for your time, and I want a functional, quality job I can be proud to show people, and tell them you did it. So as long as you are ok with that expectation, and that I won't give away my money if the work isn't done, and done right, I'll give you 50% now and the rest upon completion" The response I got was "I'll give you a call when a slot opens up"...never heard from the guy again.
This was a shop I took my car to after another shop screwed it up. This second guy had already looked at the work and agreed it was crap. Guess he didn't trust his own work enough to risk doing business with someone that would hold him to his word.
Now a buddy and I both do this ourselves, things were pretty rough looking at 1st, but functional. A few years later we do some half way decent work for ourselves.
Pisses you off to pay top buck for crap huh?