You can purchase a vehicle after you file for bankruptcy. Most Chapter 7 filers get multiple offers to finance vehicles right after they file. The interest rates will be horrible, so you want to use restraint and borrow as little as you can to get what you need.
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There wouldn't be any problem with you purchasing a vehicle, but it you need to get financing, good luck. Most lenders will not be able to qualify you for financing until after you receive your discharge.
Hope this perspective helps!
In hind sight, perhaps you should have purchased your replacement vehicle before you filed. Since you have filed, your options are limited. You should contact "722 redemption" which is a company that will finance you through US Bank at a high interest rate, but sends you to car dealers to make your best deal. Not like your local overpriced "finance here" local scammers. If you have a credit union, they may be willing to work with you if they were not hurt in your bankruptcy.