After doing some checking and calling around for parts on that first used one (U-line) I bought, I decided that one factor in buying my new one would be that it have an internal oil reservoir. The really cheap new ones ($200 or so) that I looked at had a rubber oil tank on the outside, that is easy to spot.

On my used one, the rubber was cracked with age, and leaked. It would have been easy to fix, but I could not find a place that sold the part. I called U-line and was told it was unavailable. The local fork lift repair shop told me to just buy a new one. I could have fixed it with a quart oil bottle mounted remotely, but by that time I decided on the new 21".