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They must have been dunning you for the medical bill for several months, or is this the first time that you heard about it? The first thing I would do is find out why your insurance company refused to pay the doctor's bill. With my insurance it is illegal for the doctor to bill me if my health insurance disallows a procedure. The doctor has to absorb the fee.
Did it report to all 3 bureaus? Like one said get ahold of your insurance and see what happened and sometimes they are easy to work with - with a small bill like that i would go through your credit union or bank and talk to them instead of going through a dealership for a purchase - Unless your going to lease then your score will be fine with the due at signing , whatever dealership your thinking about go an talk to them about the leasing programs they have and see what bureau they pull and the requirements , you will be surprised . Good Luck
Don't pay yet! Try a PFD with the doctor. I'm sure he would rather have the cash! Expecially if you explain the situation with wanting to buy a car and house. Sounds like an honest mistake!