Low Credit, High Down Payment. Odds Of Approved Auto Loan?

I am currently in the market for a truck upgrade. My Ford Ranger simply can't hold up to the work load it has, and what I need to be able to do (Haul apple bins, horse and flat bed trailers) It is a nice truck for its age. I do not want to ruin its value by running it into the ground.
I have relatively new credit, I have only one line open (Les Schwab tires) and a current score of 589. Not one late payment.
With the intent to sell the Ranger, and payoff the tires I plan to have a 4000-5000 down payment on the vehicle I choose. The max I will be spending on this new truck will be $9000. I plan to buy used. I don't need fancy. Just a little newer, space, and able to haul and hold up to the work load it will have. Something I can use for a while. I just feel anything newer would be out of my monthly price range. I would like to keep payments under 200. Simply to give space for emergencies.
I make around $1000 a month before tax. However, I work as a horse trainer, and do a various amount of jobs for locals that can gain me anywhere between $600-800 a month on top of my normal wage.
I would like to know my odds of an approved auto loan with the above information. I am still very green to credit, learning how it works and the best way to build/use it. Any advice is apprentiated. I just want to get some information/ideas before I start walking into banks or dealers!
Thanks!

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