I'm on a tight budget so i just bought an old (1972) D5 50j.
I looked at many dozers and decided on this because it sounded right, didnt smell of oil or hydrolic oil, didnt smoke and the track gear was in good nick.
I got it cheap because it looks scruffy as it has lived in the paddock all it life.
This is what i did, but my dozer is for farm use and if i bought to make a living, i may have done it differerently.