My signed Quote/work order has a few conditions ie: Payment COD 3-5 Buisness days, non payment after 14 liens are placed with etc. IF I don't manage to get a signed contract. Either way if it's on paper you at least have something to take to a laywer if the need arises.
What ever you use make sure you get THEIR signature somewhere on something.
UNLESS you pull a Richard Marcinco and send a dirt bag contractor trying to stiff ya a registered letter he has to sign for stating that reciept of said letter constitutes accecptence of all conditions etc...saved my ass a pile of cash last year
In most cases the dispute is over non payment and or quality in my profession.
As has been stated if the client ain't willing to cough up a sizable deposit, turn and walk.
Been burnt in the past, (nature of the beast in construction in these parts sometimes) but you can certianly take steps to avoid 99% of it.