For the amount you want to spend, you will buy junk. It will cost you twice that amount to fix whats wrong with it, depending on availablity of parts. If somebodys is selling a dozer that cheap, you know it's got issues.
Hire the pros on this one. They will get it right the first time, and be done with it before you can even get started.
Building roads aint rocket science, but understanding dirt and what it takes to stay where you want it does require some skills. Your not just hireing somebody to drive a tractor, your hireing that piece of machinery to be in working order and an experienced operator who can create drainage and compaction quickly and gurantee that it will be right.
One thing to remember when hiring operators, is to keep you own hour log if you contract the work by the hour. Let them know you are keeping track also, otherwise, you'll get some creative padding on the hours. I don't pay for down time when the tractor gets a flat because the tires are bald. Things like that. I also don't pay for lunch breaks. If they take off for two hours, then I'm not paying for it.
You have to let them know this beforehand, or you will have issues when the bill arrives.