Depends on where the dent is and how big. I have a good sized dent in my 575, but Krispy said not to worry about it (it's in the thickest part of the frame.)
Can't really help you with the rest - you know your financial situation better than any of us can.
Though personally, I'd never buy a bike online that I can't see first. I always set it up so I can pick it up and pay in person (assuming it's as described), or find one that's in an area where a friend can pick it up and do the transaction for me.
"Life should not be a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "Wow, what a Ride!"
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