Nothing.
I bought my Crown Vic for $350 with small front damage (because this car is bolted not welded together, the repair was very easy for me, an inexperienced mechanic).
Everything works, and when I bought it, the owner spent $400 on tires and brakes.
Before buying this car, my mom said we needed a 6 seat car, and when I considered having a small car and a van, I was like naaa, it will be cheaper operating a Crown Vic (insurance,oil changes,gas... etc).
I feel the Vic has a lot of life in it left. When I visited local wrecking yards, I had seen Vics with 300, 400, 500, 917 thousand miles on them, so I was convinced that I bought the correct car for my needs.
Congrats!!!
Let me say that the Crown Vic is one of the best cars out there (in my opinion), not only because it's a FORD, but because is a very solid and reliable car. You won't regret buying this car, especially when you hit the freeway!!!
Again, congratulations, and wish you the best of luck with your Crown Vic!!!